Roux
A roux is flour cooked briefly in fat, the starting point for thickening a pie filling so it stays saucy instead of watery.
Cooking the flour first does two things: it kills the raw cereal taste and coats the starch in fat so it disperses without lumps when liquid arrives.
For pies, a blond roux cooked two minutes is standard. Count on equal weights of butter and flour, then add stock in stages, stirring between each.