Add the butter to a large pot set over medium heat. When melted add the flour, whisking to combine. Let cook an additional 30-60 seconds to cook off any floury taste.
2 tbsp butter, 2 tbsp flour
Add the chicken broth 1 cup at a time, whisking to evenly combine between each pour. Once all of the broth has been completely incorporated, stir in the milk.
4 cups chicken broth, 1 cup whole milk
Add the mashed potatoes and the cheese, stirring until combined.
4 cups leftover mashed potatoes, 1/2 cup shredded cheddar cheese
Bring the soup to a simmer, then cook for 3-5 more minutes- stirring often, just until the soup's thickened to your preferred consistency. Season with salt and pepper, to taste.
salt & pepper
Serve warm garnished with shredded cheese, crisp bacon bits, and/or slice green onions.
bacon bits, shredded cheese, sliced green onions