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