Take 700 grams of karela (bitter gourd) in a large bowl. Add salt water and let them rest for 2 to 3 minutes. Then cut them into small pieces.
Mix ½ tsp of turmeric powder, 1 tsp lemon juice, and 1 tsp of salt to the karela. Cover it with another bowl and leave it for 15 minutes.
Squeeze out the water from the karela pieces.
Heat 2 tbsp of mustard oil over a high flame. When it is smoking hot, put the karela. Mix well. Reduce the flame, cover the kadai, and let it cook for 6 to 7 minutes, stirring a couple of times in between. Transfer them in a bowl when done.
Heat 3 tbsp of mustard oil again. Put 1 tsp of cumin seeds, ½ tsp of nigella seeds (kalonji), ½ tsp of fennel seeds (saunf), and ¼ tsp of asafetida (hing) in it and let them crackle.
Add finely chopped pieces of 3 medium-sized onions to it. Cook it on a high flame until soft. Then add 1 tsp ginger garlic paste, mix and cook until the raw smell is gone. Add one diced onion. Mix nicely and cook for a minute.
Reduce the flame and add ½ tsp of turmeric powder and ½ tsp of Kashmiri red chili powder. Mix well. Add 2 pieces of finely chopped, medium-sized tomatoes and salt to taste. Mix and cook on a high flame for 30 seconds. Reduce the flame, cover and cook on a medium to low flame for 3 minutes until the tomatoes turn soft.
Add ½ tsp each of coriander powder, cumin powder, saunf powder and ¼ tsp of garam masala powder. Mix nicely and cook for a minute. Add the cooked karela pieces to it. Mix nicely, cover and cook on a high flame for 6 to 7 minutes.
Your tasty and spicy homemade karela sabji is done. Transfer it on a plate and serve hot with rice or roti.