Step 1

Melt 2 tablespoons butter in a medium saucepan set over low heat. Add the onions, garlic, thyme, and bay leaf and cook until the onions are caramelized, about 35 minutes.

Step 2

Increase the heat to high then add the sherry, and cook while scraping up any browned bits, for 1 minute. Add the beef bouillon and Worcestershire sauce and stir to combine.

Step 3

Remove the mixture from the heat and discard the bay leaf. Set the mixture aside.

Step 4

Bing a large pot of salted water to a boil. Add the pasta and cook until al dente, about 10 minutes. Drain the pasta and set it aside.

Step 5

In a medium saucepan set over medium heat, warm the milk until it is simmering then set it aside.

Step 6

In a large heavy-bottomed stock pot set over medium heat, melt the remaining 8 tablespoons butter. Once the butter has melted, whisk in the flour and cook, whisking constantly, until it turns light brown in color and has a slightly toasted aroma, about 3 minutes.

Step 7

Slowly whisk in the warm milk. Continue whisking constantly to avoid any lumps.

Step 8

Cook the mixture while whisking constantly until it is thick enough to coat the back of a spoon, about 2 minutes.

Step 9

Reduce the heat to medium. Stir in the cheddar and mozzarella cheeses until they aremelted and the sauce is smooth.

Step 10

Add the pasta, stirring to combine, then fold in the onion mixture and serve.

Step 11

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