Heat oil in a pan. Add mustard seeds. When they splutter and curry leaves and fry for 2 - 3 minutes on a low heat.
Boil potato cubes with chopped tomatoes in 1/2 cup water. Add salt to taste. Cover with a lid and boil on a low heat for 15 - 20 minutes or till done ( or you can separately cook Potato cubes in Microwave - Microwave High for 3 minutes with 1/2 cup water and then mix with chopped tomatoes and cook on a low heat for 10 minutes ).
Add coconut paste, adjust salt, mix well, stir occasionally. Cover with a lid and cook on a low heat for 10 minutes.
Pour seasoning over this curry. Serve as a side dish with rice / dosa / chapati...