Preheat oven to 325 degrees.
Heat the bacon grease in a two-quart saucepot over low heat. Cook onion five to six minutes then add half and half and evaporated milk into saucepot. Bring to a simmer. In a separate skillet, melt butter and stir in flour to make a roux. Cook until the roux becomes light blond and add to milk mixture. Cook for six to seven minutes on low, stirring constantly. Remove from heat and fold in Velveeta, cheddar cheese, pepper and salt. Stir until cheeses are melted.
While you are preparing the sauce bring six quarts of water to a boil. Add one tablespoon salt and cook macaroni to just tender. Drain and fold macaroni into cheese mixture. Place in a two-quart baking dish and bake for 25 minutes. Yield: 5-8 portions