In a large Dutch oven, melt butter and olive oil over medium heat. Add onions and a pinch of salt. Cook slowly for 30-40 minutes, stirring occasionally, until deeply golden and soft. Add garlic and cook for 1 more minute.

Push onions to one side, add ground beef to the other side of the pot over medium-high heat. Brown the beef, then drain excess fat. Stir beef and onions together. Sprinkle with flour and cook for 1 minute. Deglaze with red wine, scraping the pot. Add beef broth, thyme, and bay leaf. Simmer for 10-15 minutes until thickened.

Preheat oven to 400°F (200°C). Remove the bay leaf and season the beef mixture with salt and pepper. Pour into a 9x13 inch baking dish. Top with baguette slices, then sprinkle with shredded Gruyere cheese.

Bake for 15-20 minutes, until the cheese is melted, bubbly, and golden brown. Let it rest for a few minutes before serving.