Cut the tomatoes into fine cubes. Dice 1/2 onion as well. Boil both with vinegar, sugar and mustard seeds. Let everything boil down at medium heat for about 10 minutes. Season to taste with salt and pepper.
Pluck basil leaves from the stalks, except for something to garnish, chop and stir into the chutney. Cook rice in boiling salted water according to package instructions. Season fish with salt. Cut 1/2 onion into strips and put them in a pan with orange slices.
Add stock and bring to the boil. Place the salmon fillet on the onion strips and orange slices. Cover and steam over medium heat for about 10 minutes. Drain the rice. Arrange on a plate with salmon and chutney.
Garnish with tomato wedge and remaining basil.