Boil 1 quart of lentils until tender; then heat 2 tablespoonfuls of butter. Add 1 chopped onion and stir in 1 tablespoonful of flour until brown; add some cold water mixed with vinegar. Let boil and pour the sauce over the lentils. Season with salt and pepper, and serve with small boiled sausages. Sprinkle the top with bread-crumbs fried in butter until brown.