Onion Tomato Masala, is the base gravy for many Indian dishes,you can make paneer butter masala, mushroom masala ,aloo curry & many more with this one base gravy. When we were in Bangalore, i had a chance to sneak peek into the kitchen of the restaurant where we usually brought the north Indian curries. They had the large vessel of some gravy, so depends on the order, they sauted the veggies and add the already prepared gravy & some spices. That's it ,pipping hot gravy is ready to serve. And even you can see the same procedure in any road side food stalls in India.
Adding cashews give creaminess & richness to the gravy.
If the tomato has more tart taste, add 1/2 tsp of sugar to balance the gravy.
To make the gravy even more richer, add 2 tbsp of cream or Coconut Milk at the end.
The color of gravy depends on red chili powder and the tomato, so to get bright red color, always use well ripe tomato & good quality red chili powder.