Cook the rigatoni in a large pot of boiling salted water according to package directions, drain and set aside.
Meanwhile, cook heat the olive oil in a large skillet over medium-high heat. Once hot, add the sausage and cook, breaking up with a wooden spoon until no longer pink.
Pour the sausage onto a paper towel lined plate and set aside. Wipe any excess grease from the pan and return to the heat.
Melt the butter in the same skillet you cooked the sausage in.
Once the butter is melted, add the garlic and cook stirring constantly for 1 minute, then sprinkle over the flour. Season with the Italian seasoning, red pepper flakes, and salt and pepper to taste.
Whisk the flour into the butter constantly for about 2 minutes to allow the flour to cook.
Gradually whisk in the milk, as well as the mustard and hot sauce if using, until you have a smooth and creamy sauce. At this point, taste the sauce, and add more salt & pepper to taste if necessary.
Remove the pan from the heat and stir in the cheeses, a few handfuls at a time, until smooth.
Stir in the spinach if using and cook for just 1-2 minutes while stirring constantly to wilt the spinach.
Stir in the sausage. Pour the cheese sauce over the cooked pasta and stir well to combine.
Serve immediately or make the optional topping if desired.
OPTIONAL TOPPING:
Pour the pasta into an oven proof pan (I use the same pan that we cook the sausage and make the sauce in), or a 9x13 inch baking dish.
In a medium bowl stir together the melted butter with the panko, garlic powder, Italian seasoning, and parsley.
Sprinkle the pasta evenly with the breadcrumb mixture.
Place under the oven broiler for 2-4 minutes, just until the breadcrumbs are golden.
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Notes
NOTE: It's important to use FRESHLY SHREDDED cheese off the block for this recipe! Pre-shredded cheese will not create a smooth sauce. NOTE: Once prepared, this dish is good in the refrigerator for up to 3 days, I do not recommend freezing it. NOTE: I don't recommend substituting the Gruyere or Parmesan with any other kind of cheese. If you can't find fontina, Gouda or Provolone are on OK substitute.NOTE: Don't skip the dijon and hot sauce unless you must! It adds great flavor, and brings out the cheesiness of the dish, it does not make it spicy or taste like mustard!