Preheat oven to 220 degrees. Salt and pepper the fish fillets on both sides and drizzle with lemon juice. Wash and clean the zucchini, halve lengthways and cut into thin pieces. Peel and finely dice the onion.
Heat 2 tablespoons of oil, sauté onions and zucchini, remove from heat. Stir in the hollandaise or bechamel sauce and cream. Warm up again briefly. Add cheese and 1 - 2 teaspoons of oregano. Mix everything, season well with salt and pepper.
Grease a baking dish with a little oil. Place the fish fillets side by side in the baking dish. Spread the hollandaise or bechamel vegetables on top and cook in the oven for about 15 minutes.
Serve sprinkled with finely chopped chives. Rice, pasta, potatoes and spaetzle go well with it.