Given the task definition and input, reply with output. In this task, you will be presented with the directions of a recipe separated by "," and have to fill in the "___" which is a step that is missing from the recipe.

Melt butter.,Stir in flour until well blended.,Cook and stir in milk slowly; season with salt.,Cook sauce until smooth and boiling.,Remove from heat.,Add cheese, a little at a time.,Be careful; it may curdle.,Stir constantly.,When cheese is melted, add pepper, pimento and olives.,Grease a 1-quart casserole and fill with alternate layers of potatoes and cheese sauce.,______
Bake for 1 1/2 to 2 hours at 350.