Make a light roux with the flour and butter or oil by heating the oil and flour over medium-high heat, stirring constantly.
Bring the stock to a boil. Whip the roux into the stock, making sure all lumps are gone. Simmer for 30-40 minutes, skimming the surface as needed. Strain the sauce through a chinois or strainer lined with cheesecloth. Adjust seasoning with salt and pepper to taste.