Drizzle the fish with lemon juice, chop 500 g broccoli and carrots in salted water and cook for about 8 minutes. Add the leek, also cut into small pieces, after approx. 5 minutes.
Heat 2 tablespoons butter, sweat the flour, add milk and cream, mix well with a whisk, simmer for about 5 minutes, then add the mustard, salt and pepper.
Boil the potatoes, then peel them off.
Preheat the oven to 170 degrees (hot air), salt and pepper the fish, place in a greased casserole dish, spread the vegetables over it without water, finally add the sauce - bake for about 15-20 minutes.
Cook 500 grams of broccoli in salted water for about 10 minutes (for puree), puree the broccoli with a blender, mix with creme fraiche, season with salt and pepper.
Heat the rest of the butter, fry whole potatoes in it, season with salt and sprinkle with finely plucked parsley.
Serve the fish with the broccoli puree and potatoes.