Preheat your oven to 350 degrees and prepare your pasta.
Melt 1⁄2 cup of butter. Whisk in the flour and ground red pepper and continue to whisk the mixture constantly for one minute. Gradually whisk in the milk and cook over medium heat, while whisking constantly, for six to seven minutes.
Add your spicy mustard to the mixture. Remove it from heat.
Chop the onion and add it to another stove-top pan with the cherry tomatoes. Let the mixture sauté for about 10 minutes.
Toss together the cheeses in a bowl. Reserve 1 1⁄2 cups of the cheese mixture.
Add the remaining cheese mixture and cooked pasta to the sauce, tossing to coat it thoroughly.
Pour the mixture into a greased casserole dish.
Pour your reserved cheese mixture on top.
Toss the breadcrumbs with the butter and add to the casserole.