In a large, deep skillet, melt the butter. Add the garlic, onion, bay leaf and red pepper and cook over moderate heat until the garlic is fragrant. Add the tomatoes and their juices and bring to a boil. Simmer the sauce over low heat, stirring occasionally, until thickened and reduced to 2 1/2 cups, about 1 1/2 hours. Discard the garlic, onion and bay leaf. Season the sauce with salt and keep warm.