Preheat the oven to 350. Grease a 9x13 casserole dish.
Cook pasta according to package instructions. Set aside.
In a large saucepan, melt flour and butter together and cook for 1 minute. Slowly add hot milk and whisk until thick.
Add American cheese, sharp cheddar, mustard and jalapeno. Stir together until cheese is melted and well incorporated. Add more milk if you want to thin the sauce out a bit.
Add cooked pasta to the cheese sauce. Add salt and pepper to taste.
Transfer to a casserole dish and cover with the mild cheddar.
Bake until bubbly all over, about 40 minutes.
Optional: broil until the top cheese layer is browned. Watch carefully!
Notes
This take on a Southern classic takes things up a notch with a little spicy kick.