Preheat oven to 275°. Cut salmon into 3/4-in. cubes and put in a large bowl.
Grind mustard seeds in a spice grinder or clean coffee grinder. Pour into a small bowl and mix with 1 tsp. salt, the curry powder, and cayenne. Sprinkle over salmon and turn to coat thoroughly.
Cut tomato into 1/2-in. dice and mince shallot. Cut kernels off corn cobs. Add vegetables to salmon along with oil.
Stack several basil leaves, roll lengthwise, and slice very thinly to make a chiffonade; repeat with remaining leaves. Mix 3/4 of leaves into salmon bowl.
Spread salmon and vegetables in one layer on a large rimmed baking sheet. Bake just until salmon is firm and barely flakes, 9 to 12 minutes. Add a squeeze of lemon juice if you like, and season with salt.
Spoon rice into bowls; add salmon and vegetables. Top with remaining basil.
Note: Nutritional analysis is per serving without rice.