In a Dutch oven, cook macaroni according to package directions. Drain; return to Dutch oven.
Meanwhile, in a medium saucepan cook and stir onion in hot butter over medium heat until tender, about 4 minutes. Stir in flour. Add milk, whipping cream, bay leaf, mustard, salt, and pepper. Cook and stir until thickened and bubbly, about 5 minutes. Remove bay leaf.
Add sauce and cheese to pasta in Dutch oven. Cook and stir over medium heat for 8 to 10 minutes, or until hot and bubbly. Makes 4 servings.
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