Melt butter in a 3 quart sauce pan over medium high heat. Add onions and brown sugar and cook 10 minutes, stirring often, until soft and beginning to caramelize. Add Worcestershire and sherry, and continue to cook for 10 minutes to reduce the wine and infuse the onions with flavor. Add garlic and cook 1 minute.

Add chicken broth and season with salt and pepper. Bring to a boil and add pasta. Cook for 5-8 minutes until pasta is al dente. Stir in cream. Transfer mixture to a 1 quart prepared baking dish. Stir in Gruyère. Top with a sprinkle of Parmesan.

Refrigerate at this point until baking if desired. Bake covered at 350 for 20 minutes. Uncover and continue baking for 10-15 minutes until heated through. Garnish with thyme leaves if desired.