Cook sausage in a medium skillet overmedium heat, breaking up into small pieceswith the back of a spoon, until browned andcooked through, about 5 minutes; set asideon a paper towel–lined plate to drain.
Melt butter in a large saucepan overmedium heat.
Add flour; stir until paleyellow, about 2 minutes. Gradually whisk inmilk. Bring to a boil, whisking constantly.Reduce heat to medium; continue to whiskuntil thick, about 10 minutes. Stir indrained sausage and red pepper flakes.Season to taste with salt and pepper.DO AHEAD: Can be made 1 day ahead.
Letcool completely, cover, and chill. Rewarmbefore serving.