Cut fish into 7cm wide strips and toss with salt, cayenne, 0.5 teaspoon mustard powder, and pepper.
Whisk remaining 2 teaspoons mustard powder into 300ml water to create a thin liquid.
Heat oil in a pan and fry fish skin-side down for one minute before flipping for another minute.
Remove fish and fry mustard seeds, cumin, sliced onion, chillies, and bay leaf for five minutes.
Add turmeric, garam masala, and tomatoes, then cook for two minutes.
Pour in mustard liquid, simmer for three minutes until reduced by one-third, then return fish to warm through.
Serve the fish curry immediately with the cooked rice.