What you need:
Potatoes - 2 large, skinned and cut into medium sized pieces
Tomato - 4, cut into largish pieces
Turmeric powder - 1/2 tsp
Sambar powder or red chilli powder - 3/4 tsp
Take potatoes with about 1.5 cups of water in a pan and boil until cooked, but not mushy. Add turmeric powder, sambar powder, salt and tomatoes. Boil until everything gets blended together well. The tomatoes should not turn mushy.
This is a really simple curry that goes well with rotis.