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.

In a large Dutch oven, melt butter over medium heat.,Add onion, and cook for 6-8 minutes or until tender.,______,Gradually add milk, and cook, stirring occasionally, until thickened and bubbly.,Add cubed cheese, Monterey Jack cheese, parsley, dry mustard, salt, and pepper; cook over low heat, stirring constantly until cheese is melted and mixture is smooth.,Add cooked pasta, stirring gently to combine.,Serve immediately; garnish with crumbled bacon or shredded cheddar cheese.
Add flour; cook, stirring constantly, for 2 minutes.