Wash the plaice fillets, dab dry and halve lengthwise. Sprinkle with lemon juice
Bark white bread, crumble finely. Peel and chop onion. Heat 1 tablespoon butter. Sauté onion in it. Add half the white bread. Stir in milk, bring to the boil and simmer for 3-5 minutes. Wash, peel and grate horseradish. Stir into the sauce. Season to taste with stock, salt and pepper
Season plaice fillets, roll them up and pin them down. Put 3 in 4 small greased ramekins or all in a large form. Spread sauce over them. Sprinkle remaining breadcrumbs over it. Spread 1 tablespoon of butter in flakes on top. Bake in the preheated oven (electric cooker: 200 °C/circulating air: 175 °C/gas: level 3) for approx. 20 minutes and garnish if necessary