Heat some oil in a wok, and add the cumin seeds into it. As soon as they crackle, add the onions into it. Add a pinch of salt over the onions. After a minute or two, add the ginger garlic paste and keep sauteing it. Once the onions are translucent,add the half of the sliced tomatoes/tomato puree into it. At this point, you have to taste the mixture and accordingly add the rest of the tomatoes or puree, in order to prevent it from becoming too tangy. Once you've decided how much tomato to use, cover with a lid for 2-3 mins if using puree and 5 mins if using sliced tomatoes.