Wash lemon hot, dry and finely grate peel. Halve the lemon and squeeze one half. Clean and wash the Romanesco and divide into small florets.
Dab meat dry. Bring 300 ml water to the boil in a large pan. Stir in stock. Spread the meat in it flat side by side, bring to the boil again and cook over a low heat for about 6 minutes until medium rare.
Meanwhile cook Romanesco in boiling salted water for about 5 minutes. Peel and finely dice the onion. Heat 2 tablespoons of butter in a saucepan. Sauté onion for about 2 minutes. Remove meat from the pan and keep warm. Add sour cream and meat stock to the onions, bring to the boil and simmer for about 3 minutes. Puree finely with a hand blender. Add horseradish, lemon juice and lemon peel. Season to taste with salt and pepper.
Drain the Romanesco, put it back into the pot, spread 2 tablespoons of butter in pieces over it. Season with nutmeg. Arrange vegetables, meat and sauce.
TIP: Hoof steak slices are cheaper and have a somewhat stronger taste. The preparation time remains the same.