Heat oven to 425 degrees and bring a pot of water to a boil. Once water is boiling, generously add salt.
While waiting for water to boil, trim and halve or quarter Brussels sprouts depending on size. Set them on a baking sheet. Drizzle olive oil over top and season with 1/2 teaspoon kosher salt and some pepper. Toss to evenly coat sprouts. Slice or dice (chef's choice) sausages and evenly sprinkle over baking sheet. Roast for 15 minutes or until sprouts are browned, tender and a little crispy. Sausages should be slightly browned too. When done, remove pan from oven and set aside.
While sprouts and sausages are roasting, cook the pasta until al dente. Drain and set aside.
Place empty pasta pot back on the stovetop over medium-low heat. Add 4 tablespoons of butter and melt. Add flour, dry mustard, garlic powder and cayenne to the melted butter and whisk constantly until foaming, about 3 minutes.
While whisking, slowly add milk and continue whisking until smooth. Cook for 5 minutes or until thick, whisking constantly. Reduce the heat to low.
Remove the pot from heat and add the Gruyere and cheddar cheeses, 1/2 teaspoon salt, seasoned salt, and freshly ground black pepper. Taste the sauce and add more salt and seasoned salt as needed. Be careful not to under salt or your mac-n-cheese will be a bummer!
Heat the broiler to high. Add the pasta, sausage and Brussels sprouts to the pot and stir until well-coated. Transfer to the same sheet pan as before and arrange in an even layer.
Place remaining 2 tablespoons of butter in microwave safe dish and microwave until melted. Add the pretzel crumbs and parmesan to the melted butter and toss to combine. Sprinkle over the pan of pasta.
Broil until the pretzel crumbs are golden-brown, about 3 minutes.
Enjoy!
