Step 1

Heat 3 tablespoons of olive oil and the butter in a wide braiser pan or a large Dutch oven over medium-low heat. Add the onion, 1 1/2 teaspoons of salt, and pepper; cook, stirring occasionally, until the onion is very soft and a deeply golden brown, 15 to 20 minutes. Add the mushrooms, thyme, and garlic and cook until the mushrooms begin to soften, 3 to 5 minutes.

Step 2

Stir in the wine and cook until it is almost completely evaporated, about 2 minutes. Pour in the beef broth, 3 cups of water, and Worcestershire sauce. Bring to a boil over medium-high heat. Add the pasta and cook, stirring frequently until al dente, 10 to 11 minutes. Remove the thyme sprigs. Stir in the cream cheese and red wine vinegar.

Step 3

Turn the broiler on high. Brush or drizzle the bread with the remaining 2 tablespoons of olive oil and sprinkle the remaining 1/2 teaspoon of salt. Top the pasta with the bread and sprinkle with the mozzarella and gruyere cheese. Broil until the cheese is deeply golden, 2 to 5 minutes, keeping a close eye on the pasta. Let stand for 5 minutes before serving with extra thyme sprinkled on top.