For the fish gratin, clean the fish, season, sprinkle with the juice of a lemon if desired. Leave to stand for 10 minutes, then sear on both sides in 1 tablespoon olive oil, set aside.
Preheat the oven grill. Clean the leeks, cut into narrow rings and sauté in 1 tbsp olive oil. Deglaze with a dash of white wine and cook until done in 10 minutes.
Later chop in a hand blender, fold in the crème fraîche, season repeatedly.
Coat a shallow oven-proof dish with a little leek cream, place the fish on top and coat with the leek cream.
Sprinkle the chopped kitchen herbs and the diced peppers on top and let it color under the broiler.
Arrange the fish gratin and serve.