I like a lot of sauce that I can dip naan and poppadoms into but when I try to cook a more traditional curry (I.e. without a base sauce) it always comes out too thick. Adding water though just makes it too thin and waters down the flavour?

Should I use more onions and tomato to thicken it with more spices?

A man after my own heart!

Indian curries always had a decent amount of, not too thick, gravy/sauce, because they used to be served in Katories, or small bowls. You could spoon it up with a piece of Chapatti/naan/roti. With the change over in recent years to piling everything onto one plate, and buffet meals, the dals and curries have become thicker.

I still tend to make my curries somewhere in between and provide small bowls for people to use, even at buffets. There is no reason why you shouldn't add more water during cooking so that it has time to simmer and mingle with your ingredients. Too much of thick sauce would be a turn off for me, but if you want it, you will have to increase the quantities of everything; onion/ginger/garlic/tomatoes/yoghurt/spices/salt etc.

Mamta