I'd recommend sautéing the onions in a bit of olive oil, or butter for half an hour, to 45 minutes. Onions and other aromatics need heat without too much moisture to really bring out the flavor.
You should be looking for a nice translucency to the onions, and a good glazing on the bottom of your cooking surface. Add a few halved carrots and some celery for a very nice, natural sweetness to the dish.
Google umami (sometimes spelled oomami).