Prepare the Onions and Mushrooms : Heat a drizzle of olive oil in a large skillet over medium heat. Add the pearl onions and cook for about 5–7 minutes, stirring occasionally, until they are golden and caramelized. Season with a pinch of salt and transfer to a plate. In the same skillet, add a bit more olive oil and the sliced Crimini mushrooms. Cook for 5–7 minutes, or until the mushrooms release their moisture and are browned. Add the thyme and stir for 1 minute.

Make the Sauce : In the skillet with the mushrooms, pour in the beef broth and red wine. Bring to a simmer and let it reduce slightly for 5–6 minutes. In a small saucepan, melt 3 tablespoons of butter over medium heat. Whisk in the flour and cook for 1–2 minutes to form a roux. Slowly whisk the roux into the mushroom sauce until it thickens. Add the caramelized pearl onions back into the skillet and mix well. Season with salt and freshly ground black pepper to taste.

Prepare the Baguette Slices : Preheat your oven to 375°F (190°C). Spread the baguette slices on a baking sheet. Melt the remaining 2 tablespoons of butter and mix with 1 tablespoon of minced garlic. Brush the mixture over the baguette slices. Bake for 5–7 minutes until lightly toasted.

Assemble the Casserole : Pour the onion and mushroom mixture into a large casserole dish. Sprinkle half of the grated Swiss cheese over the top. Arrange the toasted baguette slices in a single layer over the casserole. Sprinkle the remaining garlic over the baguette slices, followed by the rest of the Swiss cheese.

Bake and Serve : Bake the casserole in the preheated oven for 15–20 minutes, or until the cheese is bubbly and golden. If desired, broil for an additional 1–2 minutes for extra crispiness. Remove from the oven, let cool for a few minutes, and serve warm. Enjoy this comforting and flavorful casserole as a main or side dish.