Heat a medium size deep skillet over medium heat. Add oil and butter. When butter melts into the oil, stir in flour. Gently cook, whisking flour and butter together until smooth and flour has had a chance to cook, about 3 minutes. Slowly add milk while continuing to whisk. Gently bring milk to a bubble while stirring frequently. Allow the milk to thicken a bit; then stir in 2 cups of shredded cheddar cheese, a handful at a time. Season sauce with nutmeg (and/or mustard) and cayenne. Taste and add a little salt, if you like.
Add cooked pasta to the sauce and coat completely by turning over and over in the cheese sauce. Transfer to a baking dish, top with remaining cheese and place under a hot broiler for a minute to brown the top. Makes 4 servings.