Now we can get this party started! In a nice big cast iron pot or whatever oven safe pot you got, melt 4 tablespoons of the whipped butter on medium heat. Add the flour, one tablespoon at a time, whisking with each addition to combine the flour and butter to make a roux (sauce thickener). Now add the milk one cup at a time as well, whisking again with each addition. Now the arm workout comes in. We want a smooth sauce so keep on whisking until you achieve a no lump sauce. When you are lump free, add the mustard powder, cinnamon, bay leaf and onion. Stir those ingredients together, turn the heat up a smidge and boil for about 2-3 minutes. Salt and pepper to taste. After you have boiled the sauce to thicken it, lower the heat to medium-low and add that glorious cheese, stir to melt and simmer for about 5 more minutes.
