Wash the fish fillet, dab dry and cut into two equal pieces. Sprinkle the fish with lemon juice, salt it and let it stand for 5 minutes. In the meantime, clean and wash the tomatoes and spring onions.
Cut tomatoes into slices, spring onions into rings. If necessary, dab the fish fillet slightly dry and layer together with the tomatoes and spring onions in an ovenproof, greased dish. Season with salt and pepper, pour mustard sauce over it and bake in the preheated oven (electric cooker: 225 ° C/ gas: level 4) for about 20 minutes.
Serve garnished with dill. Rice tastes good with it.