Paul Prudhomme's Cajun Sauce for Beef

Combine onions, bell peppers and celery in a small bowl & set aside while you start the roux.
In a heavy 2-quart saucepan (I prefer a castiron skillet), heat the oil over medium-low heat to about 250°. With a metal whisk, whisk in the flour a little at a time until smooth.
Continue cooking, whisking constantly, until roux is light brown, about 2 to 3 minutes. be careful not to let the roux scorch or splash on you.
Remove from heat and immediately stir in the vegetable mixture (with a spoon) as well as the red, white and black peppers.
Return pan to high heat and cook about 2 minutes, stirring constantly (you're cooking the seasonings and vegetables in the light roux and the mixture should be pasty). Remove from heat.
In a separate 2-quart saucepan, bring the stock/broth to boil. Add the roux mixture by spoonfuls to the boiling stock, stirring until dissolved between each addition.
Bring mixture to a boil, then reduce heat to a simmer & cook until the sauce reduces to 3 1/2 cups (about 15 minutes).
Skim any oil from the top and serve immediately.