How much oil should you use when stir frying a single portion

As you suggest, there are a number of factors, but the best rule of thumb I've found for pan-frying is to use just enough oil to cover the bottom of the pan by swirling it around. In general, you don't want puddles (you'll get splashed with hot oil), but if the pan isn't covered then your food might not get cooked evenly.

These days I mostly just eyeball it, and err on the generous side. Using slightly too much isn't likely to hurt your dish - unused oil just won't get absorbed - but using too little will result in under-cooked or inconsistently-cooked food and probably a lot of gunk stuck to the bottom of your pan. So if you're not sure, just add a little more than you think you absolutely need.

I would do it the other way round, I'd fry the sausages first, then add the veg. This has a few benefits as I see it:-

• The sausages will brown more evenly, purely aesthetic but some people will think they are not cooked if they are not brown.
• You'll get the oil out of the sausages so you'll have a better idea of how much oil to add when you add the veg, if any.
• The veg will sweat down in the sausage oil which is likely to be much nicer than the other oil
• You can cook the sausages on a really low heat to start with so that they really get sticky and caramelised, then when you add the veg back in you can add a spoon of water/vinegar/stock to deglaze the pan an get that lovely sticky sausage goodness all over your veggies, which will be delicious.

You could optionally take the cooked sausages out of the pan and do the veg separately, or keep 'em in there, I'd probably take them out so I could turn the heat up a bit without having to worry about the sausages.

I'd say that when you do add the veg you want a thin (1mm ish) layer of oil in the pan. Depending on the pan this is probably a tables spoon or 2. You probably don't need to scale up as if you are cooking a bigger batch you will probably use a bigger pan, and so a similar depth of oil should suffice. If you are using the same pan, but with a deeper amount of veg, then I would add a little more, but I would make the depth of oil in the pan proportional to the depth of veggies.

Remember the golden rule. You can always add, you can never take away. And once the veg start to cook down and the volume decreases then the oil ratio will be higher, so start out with a little less than you think, just enough to coat the veg when tossed in it.