Sunflower lecithin (sorry, best link I could find) is gaining in popularity as an alternative to soy lecithin because it is widely perceived to have a neutral taste and actually has superior emulsifying properties. It's a little on the expensive side, though.
Soy lecithin by itself doesn't taste horrible if you buy it as a food additive (as opposed to a nutritional supplement). It's common to find in a lot of popular frozen pasta-and-sauce dishes, such as Michelina's, which obviously must be microwaveable without the sauce curdling or separating.
Another option is Sodium Stearoyl Lactylate, if you can get hold of it. This one adds a little sweetness which is generally pleasant-tasting, or at least inoffensive. You'll find it used most often as a dough conditioner or in baked goods, but another popular use for it is in commercial sauces, although unfortunately I can't find specific examples right now.
But probably the best emulsifier for what you're trying to do here is xanthan gum. It's also used as a gelling agent, but in small quantities works as just an emulsifier. You can find it, for example, in this Bertolli Creamy Alfredo Pasta Sauce - exactly the sort of thing you're making. It's also used in the Stouffer's Alfredo sauce.
Mix in a little lecithin or xanthan gum as an emulsifier while the sauce is fresh or cooking and it's very unlikely that your sauce will separate, either in the fridge or in the microwave.
As for what you can do to specifically prevent separation while reheating, if you didn't add any emulsifiers in the first place... I really don't think there's anything you can do. If you're a slow enough eater the sauce will eventually separate right in front of you; that's just what happens when you have water and fat in the same sauce. Best to reheat on the stove top in that case, and stir frequently to prevent any further separation.
I made a sauce using the ingredient ratios specified in the question. The sweet and sour in this sauce actually balanced each other and I thought this might invalidate any experiments until I considered that they also dominated the flavour of the sauce. In other words the sauce was a little too tangy (or tart if you prefer).
For the purposes of this answer, I'll make the following distinctions:
- sour: A taste where there is not enough sweetness to balance the acidic notes;
- tangy: A taste where sweet and acidic notes are balanced;
- tart: A taste that is tangy, but the sweet and acidic notes together dominate flavour.
Note that I don't suggest that this is the accepted nomenclature. I think it is possible that the OP confounded sour with tart which may explain the specification of no additional sugar in the question.
Colour as a sweetener
As the Wikipedia article on Sweetness says,
The color of food can affect sweetness perception. Adding more red
color to a drink increases its perceived sweetness with darker colored
solutions being rated 2–10% higher than lighter ones even though it
had 1% less sucrose concentration.
The colour of this sauce became much duller during cooking, so I tested the effect on taste of using a green food dye. I didn't notice any difference in a side-by-side taste-test, even though the sauce with food dye did look much nicer,
I did one more test adding more dye and this time, to my surprise, the sauce with the more vibrant colour tasted very slightly, but nevertheless distinctly sweeter than the original.
I didn't quite believe this, so I taste tested this several times with the same result.
This led me to suspect the food dye itself and, sure enough, it tasted sweet due to an amount of glycerol in the ingredients. Despite invalidating my experiment, glycerol is neither sugar nor honey and as such answers the OP's question directly (if indeed the sauce was sour and needed sweetening). Adding to that the fact that the sauce now looks more appetising, I can recommend using green food dye whether or not colour has any effect on taste.
Salt
I also tested the suggestion of another poster, where salt is used to sweeten the sauce. I didn't hold out much hope for this technique, having experimented with salt on all manner of fruits a couple of days back.
Once again, I was surprised. Salt made a positive difference, although this time the sauce was no sweeter than before. Quite the opposite; sweetness was reduced (which is consistent with my earlier experiments). The sour note was also reduced, so sweet and sour were still balanced leaving the sauce tangy but not tart.
I adding a pinch more salt and now the salty taste came through and ruined the sauce.
I recommend adding salt if the sauce is similar to mine and excessively tart rather than actually sour. It is easy to overstep the mark when adding salt. I suggest making a little more sauce than actually necessary and putting an amount aside before adding the salt so that you can re-introduce some if you do go overboard.
Dilution
Finally I tested the advice to thin-out the sauce with water. This was the least promising result and it seems to me that sweetness disappeared faster than the sour taste. I tried various dilutions, but nothing compared well to the original and flavour was lost.
Possibly this could work if you dilute the sauce with something more flavourful than water, but I would recommend the other two methods over this if you want to stay true to the original recipe.
Best Answer
You want emulsification.
Emulsification is the breaking up a fat and dispersing it into a liquid (or vice versa, dispersing a liquid in a fat). A classic example of an emulsification, also known as an emulsion, is mayonnaise.
There are at least two good ways you can emulsify your brown butter, soy sauce, and lemon juice. One way is called shearing, which is just what it sounds like. You're basically cutting off slices of droplets, making them smaller and smaller. The smaller the microdroplets, the more stable the emulsion. You can accomplish a short-lived but great textured emulsion with just energetic whisking of your liquid and fat ingredients. You can make the micro-droplets even smaller and your emulsion more stable by using a blender or immersion blender.
A second thing you can do to bind your liquid and fat ingredients into a much firmer and more stable emulsion is to add an emulsifier, an egg yolk being a very solid candidate for the job. Energetically whisk in about half of an egg yolk while slowly bringing the cool sauce to just above perfect serving temperature. If you like where the sauce is going, and would like it to be even more like that, temper, then add the other half of the yolk. Mustard is pretty effective too, for a different flavor.