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 1.5 cups lentil liquid. Stir until it thickens slightly. Add half cup catsup. Pour over the lentils in the casserole. Bake for 40 minutes. Serve hot.