Preheat oven to 350° and grease a 9 x 13 baking dish. Set aside.
Cook pasta according to package directions until slightly under cooked, al dente.
Drain and rinse with cold water.
In a large dutch oven, melt butter over low heat.
Add onions an garlic. Cook for a minute or so.
Add flour, whisk until smooth, stirring constantly.
Continue whisking while adding milk, heavy cream, vegetable broth, ground mustard, seasoned salt, cayenne pepper, salt and pepper, until thickened.
Once thickened, add cheese (save some for topping). Stir until well combined.
Pour into prepared baking dish.
Top with remaining cheese.
Cook uncovered for 25 minutes.