Melt the butter in a saucepan, add the flour while stirring, add about 300 ml of warm boiled water, stirring constantly with a whisk, bring to a boil. Take it off the heat, add horseradish, season with salt and pepper, add cream, mix. In a frying pan in butter, fry the halves of the mushrooms until golden brown, add the onions, fry them together for a while, season with salt and pepper. In a frying pan, fry the loop of previously scalded white sausage. In an ovenproof dish, pour 3/4 cup of the horseradish sauce to the bottom, arrange the mushrooms, onion and sausage, and pour over the remaining sauce. Bake in the oven for about 20 minutes. Before serving, garnish with lettuce.