Soak the mustard, cumin seeds, and dry red chilies in water for 20 - 30 minutes. Remove from water and transfer to a blender jar. Also, add garlic and tomatoes to it. Grind them together to a fine paste. Add a little water only if required.
In a pan, heat oil, add cumin seeds, and let it crackle. Then add a pinch of hing, some more crushed garlic or ginger garlic paste, and green chilies and saute. Then add onion and saute till onion is translucent.
Add the spices ( ground coriander, cumin, turmeric, red chili powder). Saute for 10 seconds, add chopped vegetables and mix well.
Add salt and saute for 2 minutes.
Add 1 cup water and cover with a lid. Let it simmer, and boil the vegetables till they are cooked. It will take 7 - 10 minutes.
Then, open the lid and check with a spoon if the vegetables are cooked. Add the mustard and tomato paste to the curry and mix well.
Let it simmer for 5 minutes more.
Once done, turn off the gas, add a pinch of garam masala and dry mango powder, and stir well.