Wash the fish, dab dry and cut into 8 pieces. Sprinkle fish with lemon juice. Put rice in plenty of boiling salted water, cover and let it swell at low heat for about 20 minutes.
Add the peas after about 15 minutes, cook along. Place the fish in an ovenproof dish. Spread 40 g herb butter in flakes over it. Bake in the preheated oven (electric cooker: 225 ° C/ gas: level 3) for about 20 minutes.
Peel and finely dice the onion and fry in the remaining herb butter until translucent. Add tomatoes, 1/8 litre water and stock. Let it boil down a bit and season with salt, pepper and sugar. Serve everything.
Garnish as desired with parsley and lemon wedges.