Onions! To say I'm a slow learner is an understatement, so when I first started cooking curry about 30 years ago, I thought, heat some oil cook a few chopped onions for a few minutes then chuck in your spices tomatoes and meat, cook for a while… Hey presto! Curry is served…… Wrong!!
One of the most important ingredients in a curry is the humble onion, treat it with respect! Depending on the recipe you are using, onions should be cooked longer for red meat and slightly shorter, for say, chicken, typically 15-20 minutes of browning on a medium heat, adding water if the onions start to catch on the pan. Browning the onions well adds a rich depth of flavour to the curry. I recently did a biryani dish and the onions were cooked in unsalted butter for over 30mins and they caramelised giving a really fantastic flavour to the finished dish. So, for the perfect curry, have a little patience with those onions, seriously, it'll be worth it.