Prepare Bitter Gourds: Wash 500 gm bitter gourds thoroughly in water. Rub them well to remove soil and impurities. Cut both ends and slice them in half. Remove the seeds carefully.
Salt and Rest Bitter Gourds: Place the cut bitter gourds in a bowl. Add 1 teaspoon salt, 1/2 teaspoon turmeric powder, and 1 teaspoon lemon juice. Mix well by hand and let it rest for 15-20 minutes, shaking once or twice.
Squeeze Out Bitterness: After resting, squeeze the bitter gourds to remove excess water and bitterness. This can be done by hand without adding water.
Fry Bitter Gourds: Heat mustard oil in a pan over high heat. Add the squeezed bitter gourds and fry, turning occasionally until they turn red and become soft, about 14-15 minutes. This frying further reduces bitterness.
Cook Tomatoes: In the same pan, remove half the oil leaving about half a ladle. Add 2 tomatoes and cook for 4-5 minutes until water evaporates and oil separates.
Add Onions and Chillies: Add 3 sliced onions and 4 green chillies cut lengthwise. Fry onions for 2-3 minutes until they turn pink but not fully cooked.
Add Spices: Add 1/4 teaspoon turmeric powder, 1/2 teaspoon red chili powder, 1/4 teaspoon Kashmiri red chili powder, 1/2 teaspoon coriander powder, and 1/2 teaspoon garam masala. Stir and cook spices for 1.5 minutes.
Combine Bitter Gourds and Cook: Add the fried bitter gourds to the pan, mix well, cover, and cook for 5-7 minutes.
Add Mango Pulp and Finish: Add 2 teaspoons raw mango pulp or amchoor powder at the end to remove any remaining bitterness. Garnish with chopped coriander leaves and mix.
Serve: Serve the dry bitter gourd and onion vegetable hot with paratha or roti.