Cut each potato into two pieces. Boil potatoes until soft in a pressure cooker. Peel and mash.
Heat oil. Add mustard seeds to temper. Then add chopped onions, turmeric powder and salt to taste. Cook until onions are soft. Add finely chopped green chillies or chilli powder and mashed potatoes . Remove from heat.
Cool. Add grated cheese and mix.
Cut the capsicums into two pieces each vertically after partially removing the stem. Discard the pith and seeds. Fill with the potato mixture.
Mix 1/2 teaspoon salt with 1 cup water and pour into a flat oven proof dish. Place stuffed capsicum with the filling on top and the capsicum side touching the salt water at the bottom of the dish. Bake in a hot oven for 30 minutes until the capsicum is cooked and the filling is golden brown. You can brush the filled capsicum halves with a bit of melted butter or oil if you want to before baking.
Transfer to a serving platter.
Recipe Notes
Chillies and chilli powder are optional since the capsicum has a natural spicy taste.