Bring a large pot of water to a boil. Add macaroni, salt, and a dash of oil. Cook according to al dente package directions. Drain and set aside.
In a large cast-iron skillet or Dutch oven over medium heat, brown sausage rounds in oil, about 7 to 10 minutes. Set sausage aside and quickly wipe oil from skillet. Melt butter in same skillet over medium heat. Sprinkle in the flour and whisk constantly, until flour begins to smell nutty (1 to 2 minutes). Gradually whisk in the half-and-half, dried onion, ground mustard seed, and black pepper. Continue whisking until the mixture bubbles and thickens (about 2 minutes). Slowly add cheese by the handful, letting it melt before adding the next, stirring constantly. Mix in cooked macaroni, then browned sausage, cooking a minute longer to heat through. Garnish with parsley (optional) and serve hot.
Per serving: 496 calories, 31 grams fat (18 grams saturated fat), 86 milligrams cholesterol, 499 milligrams sodium, 34 grams total carbohydrates, 1.5 grams fiber, 21 grams protein.
