Mac & Cheese w/ spinach, red onion, and Italian sausage

Got this recipe from the October 2011 edition of Everyday Food. In a large nonstick skillet, heat 1 tablespoon extra-virgin olive oil over medium. Add 1 medium red onion, halved and thinly sliced (or diced), and cook, stirring occasionally, until softened. Add 1/2 pound sweet Italian sausage and 1/2 teaspoon red pepper flakes and cook, breaking up meat with wooden spoon, until browned and cooked through. Add 4 cups macaroni & cheese (recipe below) to skillet, gently stirring to combine. Stir in 2 tablespoons of heavy cream and 5 ounces baby spinach (about 5 cups), and cook until spinach begins to wilt, 4 minutes. To serve, sprinkle with shredded Parmesan.

Note: I used all of the macaroni & cheese, not just 4 cups.

Three-cheese Macaroni

Coarse salt
1 pound elbow macaroni
5 tablespoons unsalted butter
6 tablespoons all-purpose flour
4 cups whole milk, room temperature
3/4 teaspoon ground mustard
1/2 teaspoon Old Bay seasoning
8 ounces white cheddar, shredded (3 cups)
4 ounces Monterey Jack, shredded (1 1/2 cups)
1/2 ounce Parmesan, grated (1/4 cup)

1. In a large pot of boiling salted water, cook macaroni according to package instructions. Drain.
2. In separate pot, melt 4 tablespoons butter over medium. Whisk in flour and cook, stirring frequently, until mixture is pale golden and has a nutty aroma, about 4 minutes.
3. Whisking constantly, slowly add milk. Cook, whisking along bottom of pot, until boiling, about 7 minutes. Reduce heat and gently simmer, stirring occasionally, until sauce is thickened, 10-12 minutes. Remove from heat and add 1 tablespoon butter, mustard, Old Bay, and cheeses. Stir until cheeses are melted and sauce is smooth; season with salt. Pour sauce over macaroni and stir to combine.