Wash the lentils and drain them. Pour the lentils into a large pot. Add the whole onions, bay leaf, thyme and the peppercorns. Add boiling water to cover. Bring to a boil. Reduce the heat to a simmer. Cover. Cook 30 minutes. Remove the onions, bay leaf, thyme and peppercorns. Drain, reserving liquid. Add salt & pepper. Mix the cooked lentils with sliced sausage and minced onion. Place in a buttered casserole. Preheat oven to 350 degrees. Melt the butter in a saucepan. Add the flour. Stir until smooth. Gradually add 1 1/2 cups lentil liquid. Stir until it thickens slightly. Add 1/2 cup catsup. Pour over the lentils in the casserole. Bake for 40 minutes. Serve hot.