Preheat oven to 350 degrees. In a skillet, melt the butter over medium heat. Add the onion and simmer until soft.
In a large bowl, mix the egg with the onion, salt, pepper, and thyme. Add the ground meat and stir well. Fold in the bread crumbs, cheese, and 1/2 of the tomato sauce. Form the mixture into a loaf and place it in a roasting pan Cover with the rest of the tomato sauce and the bay leaf. Bake for one hour.