Cook the pasta and broccoli. Cook macaroni in a large pot of salted boiling water until al dente, according to package instructions. During the final 3 minutes of cooking time, add the broccoli florets to the boiling water with the pasta. Drain and set aside.
Brown the sausage and onion. While the pasta and broccoli cook, heat 1 tablespoon olive oil in a large cast iron skillet or Dutch oven over medium heat. When the oil shimmers, add the sausage rounds and diced onion. Cook, stirring regularly, until the sausage is browned and the onion is tender, about 5 to 7 minutes. Remove the sausage and onion from the skillet and set aside on a plate. Wipe out the skillet.
Make the cheese sauce. Add butter to the skillet and cook, stirring, until melted. Sprinkle in the flour and cook, stirring constantly, for one more minute. Gradually whisk in the half-and-half. Cook and stir until the mixture bubbles and thickens, about 2 minutes. Reduce the heat to low, then add the cheddar, gouda, and dry mustard and stir until the cheeses melt.
Toss and serve. Add the cooked macaroni, broccoli, sausage, and onion back to the skillet and toss to combine. If the sauce seems too thick, stir in a splash or two of extra half-and-half or milk to loosen it. Season with salt and pepper to taste and garnish with parsley. Serve immediately.