Make the Mustard Paste
Soak mustard seeds in warm water for 15 mins. Grind with garlic, green chili, and a little water to make a smooth paste.
Boil the Potatoes
Parboil potato cubes with a pinch of salt. Set aside.
Fry the Crabs
Heat mustard oil till it smokes. Add a pinch of salt and turmeric to the crabs, then fry until they turn reddish and aromatic. Remove and keep aside.
Sauté the Base
In the same oil, add panch phoron or mustard + jeera. Add onions, sauté until golden.
Add tomato, turmeric, chili powder, and salt. Cook till oil separates.
Add the Mustard Paste
Stir in the mustard-garlic paste and cook for 2–3 mins on low flame.
Simmer the Curry
Add 2½ cups of water. Add potatoes and fried crabs. Cover and cook for 10–15 minutes till flavors blend.
Finish ' Serve
Let it rest 5 mins. Serve hot with steamed rice, sliced onion, and lemon.