Caramelize the Onions: In a spacious skillet, melt 4 tablespoons of butter over medium heat. Toss in the sliced onions and sauté, stirring occasionally, until they become tender and golden brown, which should take around 25-30 minutes. Then, mix in the minced garlic, fresh thyme, and a bay leaf, cooking for an additional 1-2 minutes until the aroma fills the air.

Deglaze the Pan: Pour in the dry sherry or white wine and stir to deglaze the pan, scraping up any browned bits. Stir in the beef bouillon paste and Worcestershire sauce, then cook for an additional 3-5 minutes until the liquid is reduced and absorbed by the onions. Remove the bay leaf and set the onions aside.

Cook the Pasta: As the onions caramelize, fill a large pot with salted water and bring it to a rolling boil. Add the pasta shells and cook according to the package directions until they reach an al dente texture. Drain the pasta and set it aside.

Prepare the Cheese Sauce: In the same pot used for the pasta, melt the remaining 6 tablespoons of butter over medium heat. Whisk in the flour and cook for 1-2 minutes to form a roux. Slowly whisk in the milk, making sure to eliminate any lumps. Continue whisking until the sauce thickens, about 5-7 minutes.

Add the Cheese: • In a separate pan, whisk in the flour and cook for 1-2 minutes to create a roux. Gradually add the milk, whisking continuously to ensure a lump-free mixture. Season with salt and pepper to taste.

Combine the Pasta and Cheese Sauce: Add the cooked pasta shells to the cheese sauce and stir to coat the pasta evenly. Fold in the caramelized onions, ensuring they are evenly distributed throughout the mac and cheese.

Serve: Serve the French Onion Mac and Cheese immediately, garnished with extra thyme if desired. For an added layer of texture, you can also top the dish with toasted breadcrumbs or serve it straight from the skillet for a rustic, comforting presentation.