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.
While the pasta and broccoli are cooking, heat 1 tablespoon of olive oil in a large cast iron skillet or Dutch oven over medium heat. When the oil shimmers, add the sausage rounds and the diced onion. Cook, stirring regularly, until the sausage is browned and the onion is tender, about 5-7 minutes. Remove sausage and onions from the skillet and set aside on a plate. Wipe out the skillet.
Add butter to the skillet and cook, stirring, until the butter melts. 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; add the cheddar, gouda, and dry mustard; stir until the cheeses melt. Add cooked macaroni, broccoli, sausage and onion back to the skillet; toss to combine. Garnish with parsley.
Notes
To cut down on the prep time, I like to get the pasta boiling while I prepare the rest of the ingredients (such as grating the cheese and browning the sausage and onion). Then, as the pasta finishes cooking, you can whisk together the cheese sauce. This way the pasta isn't sitting around getting cold while you prepare the sauce, and everything comes together quickly at the end.
Salt the water before boiling your pasta. This is your chance to season the pasta for a flavorful dish!
It's important to use blocks of cheese that you grate by hand. Do not use pre-shredded cheese in a package, because those packaged shredded cheeses include a powdery coating with stabilizers that prevent the cheese from melting smoothly.
The recipe calls for sharp cheddar cheese and smoked gouda, but you can use just about any good melting cheeses that you prefer. For instance, mild cheddar, gruyere, fontina, Colby, and Monterey Jack would all work well. You can even throw in some (gasp!) American cheese -- good old Velveeta! -- if that's what your family enjoys.