Getting rid of raw onion taste without the frying process

I'm trying to find a reliable way to make a good base gravy for British Indian Restaurant (BIR) curry — for those that don't know, this is essentially the adapted form of Indian curry cooking used in Indian takeaways in the Western world (I know, glamorous!).

Base gravy is essentially some alliums and sometimes tomatos / carrot / celery cooked for a long time and then blended to a soup-like consistency (after this, spices and other flavorings are added to achieved a finished dish). The basic sauce of the curry world, essentially.

The key determinant of success seems to be sweetening the onions. I can carmelize them using the standard technique (frying and deglazing), but I'd ideally like something that only uses boiling because it is less work (also, adding a lot of oil to base gravy and then again in the finished dish – to temper the spices – seems unhealthy to me).

And even in the more varied base gravy recipes in which carrots/cabbage/tomatoes have been involved, these ingredients have only been introduced towards the end of the cooking process after a base of well-sweetened onions has already been achieved.

TL;DR Can brown / sweet / onions that don't taste raw be achieved without frying?

Even if you start with frying, can the sweetening process continue to take place while the onions are boiling (once cooking temperatures are below what would be required for the Maillard reaction to take place)?

And if so, how long would the onions need to be boiled to achieve reliable results?

I notice that some base gravy recipes call for boiling the onions for very long periods of time (2 hours) but I didn't achieve what I expected using a slow cooker.

(I feel a little stupid even asking this, but take solace in the fact that learning to produce tasty, consistent base gravy is apparently something that it takes every Indian chef a while to master.)