Wash and cut capsicum into medium size square pieces.
Then in a bowl, add turmeric powder, coriander powder, red chilli powder, garam masala, kasuri methi, salt, and buttermilk. Mix well and masala paste is ready. Keep it aside.
Now, in a pan add 1 tbsp oil, capsicum pieces and salt. Sauté it on medium-high flame till it becomes slightly soft. Keep it aside.
Then in the same pan, add 3 tbsp oil, cumin seeds, and a pinch of hing. Sauté it.
Now add chopped onion and sauté till it becomes slightly transparent.
Add ginger-garlic paste and sauté it.
Now add gram flour (besan) and sauté till it slightly changes its color.
Then add chopped tomatoes and salt. cover and cook till tomatoes become soft and mashy.
Now add the masala paste and sauté till oil separates from its sides.
Then add some water and cover and cook the gravy for 2-3 minutes.
Now add capsicum pieces and mix well.
Cook curry for 2-3 minutes on low flame.
Switch off the flame and garnish with coriander leaves.
Serve shimla mirch ki sabzi with paratha, roti or rice.