Cook the macaroni and cheese according to the package directions. Drain and set aside. In a large skillet, heat the olive oil over medium-high heat. Add the onion and cook until softened, about 5 minutes. Stir in the milk and salt and pepper to taste. Cook, stirring frequently, until the mixture thickens, 2 to 3 minutes, scraping up any browned bits from the bottom of the skillet. Pour the sauce over the pasta and stir to coat. Serve immediately.