Clean and wash broccoli and cut into florets. Steam in boiling salted water for approx. 5 minutes and drain. Wash the fish, dab dry and cut in half lengthwise. Sprinkle with 2 tablespoons of lemon juice and season with salt.
Spread with horseradish, roll up and pin. Heat the fish stock and let the rolls simmer for about 5 minutes at low heat. Take out and let the stock boil down to half.
Peel and finely chop the onion. Fry in hot fat. Deglaze with wine, 4 tablespoons lemon juice and fish stock. Let it boil down a little and stir in double cream. Whisk a little sauce and egg yolk, stir into the sauce and season with salt and pepper.
Place the fish and broccoli in an ovenproof dish, pour the sauce over it and bake under a preheated grill until golden brown.