Cook the macaroni according to package directions. Drain.
Preheat the broiler. In a medium broiler-safe skillet cook sausage in 2 Tbs. hot oil over medium heat until browned, about 2 minutes. Transfer sausage to a bowl.
Add the flour to the skillet. Cook, stirring, until flour turns tan, 2-3 minutes. Whisk in the milk. Cook, stirring, until thickened and bubbly. Stir in the cheese and mustard. Stir in the macaroni and sausage. Broil 3-5 minutes, until top is browned.
Toss the lettuce and apple with the remaining 2 tsp. olive oil, cider vinegar, and salt. Serve with the mac & cheese.