You can make these a day or two ahead and store in the fridge. If baking cold from the fridge, let stand 30 minutes first. Mix well and place in a large casserole dish. If making ahead, refrigerate. Otherwise, cover and bake at 325 degrees for 45 minutes.
Gravy becomes lumpy when flour particles become stuck together.
This can be avoided by cooking together equal parts of flour with a fat such as butter or vegetable oil. This mixture is known as a roux and serves as a thickener for gravy. Cook gently over medium heat and stir constantly until all flour particles are coated and the flour becomes golden in color. Once the pan and roux have cooled, whisk in 4 cups broth. Stir and simmer over medium heat, stirring frequently for 5 minutes.