Shorshe maach

Discard old mustard seeds or you'll create an abomination. You may use shrimp instead of fish . The recipe is from my sister-in-law Barnali di.

1. Rub salt and turmeric to the fish pieces and shallow  fry very lightly in hot oil. Keep aside. Let them drain in paper towels
2. Grind yellow mustard seeds, salt, green chilis (and posto optional) with a little water. Add water in small portions if the paste is too thick to move inside the food processor.
3. In a wok, heat oil add kalonji seeds and slit green chilies 
4. Add the mustard paste, turmeric, water. Bring to boil, add salt to taste and a pinch of sugar.
5. Add the shallow fried fish.
6. When it is about done, add 1 tsp mustard oil. Add cilantro leaves if you want.

If you like , you may add a bit of red chili powder.

I made buck Shad with mustard paste. For this, I microwaved the mustard seeds in a cup of water for a minute.The seeds soaked up the water and I made a paste by slipping it into magic bullet with a few green chilies. (I usually dry grind mustard seeds in a coffee grinder. )

Chicken in mustard and tomato sauce

1. I marinated the chicken pieces(thighs with bone cut in halves with kitchen shears) in mustard sauce and vinegar overnight. To make it extra spicy, you can add slit Thai chilis.
2. Shallow fry the chicken in mustatrd oil so it isn't pink anymore.
3. Remove from oil and drain. Use the same oil to fry onions and diced tomatoes. Add red chili powder, a little plain yogurt and cook well till oil separates. (You can bake the whole think with very litle oil, but I am going the curry way)
4. Serve with freshly chopped cilantro.

See also, Slow Cooker Chicken Stroganoff [allrecipes]

And this, for low-fat meal ideas

Catfish in mustard sauce

Ingredients:
Catfish steaks
Nigella( kalonji/ onion seeds/ kaalo jeere)
Green chillis slit in half
Diced onions
Mustard powder/paste (preferably fresh. Old ones turn bitter)
Kasundi if you like

Salt, turmeric powder, oil for cooking
Cilantro for garnishing

Method:

1. Wash the steaks well, and wipe off excess water with paper towels by dabbing them.
2. Rub salt and turmeric powder and lightly fry them in oil, turning only once. Frying seafood or even freshwater fish tend to make them rubbery/chewy
3. Take out the fish, and fry the kalonji and onions. Add chilies taking care not to fry them too much, and immediately after
4. Microwave a bowl of water and add the mustard paste/powder and put in the pan. 
5. Let it simmer, add the fish steaks. Cook covered.
6. Garnish with cilantro and serve

You might want to add a little cumin powder if you like to counter the typical smell of the fish. Also, some might want to add potatoes or tomatoes. Usually, I don't.

Alternately, you can skip the mustard , use dry roasted cumin-coriander powder, potato wedges and cauliflower florets to make a light curry. Make sure the fish is really fresh for a tasty broth. I prefer live ones. Catfish is inedible if it isn't fresh. Something I had a very bad experience at a New Orleans restaurant where I ate on a whim.

Nevertheless, Cajun recipes have interesting options :
Link 1