Stephan, I use mostly grapeseed or peanut oil, about a 3-4 tablespoons to start the chopped onions with, without overcrowding the onions, I use 3 med onions.
The oil doesnt evaporate much so the grapeseed oil gets infused with the onion oil, when the chopped onion dissapears to almost nothing pour into the curry base or into a bowl, then start another batch, and repeat until you get tired of keeping a eye on it.
Just dont let it burn or as you already know, there will be a bitter taste.
Jerry Griffiths