Brown the sausage in a large skillet on medium heat until cooked through. Using a slotted spoon, remove the sausage from the pan leaving the drippings behind. Add the butter to the pan & melt, scraping up any browned bits with a whisk. Add in the flour once the butter is melted & stirred into the drippings and give it a good whisk to combine. Let the flour/butter mixture cook for about 60 seconds, then pour in the milk. Gradually whisk in the milk, reduce heat to low, & cook until thickened - about 10 minutes. Season to taste with salt & pepper. It's also really good with some crushed red pepper flakes on top if you like a little kick!