1. Begin by preheating your oven to 425 degrees Fahrenheit. Line a rimmed baking sheet with aluminum foil, ensuring easy cleanup later. Spray the foil with cooking spray to prevent sticking.
2. In a large cast iron skillet, add the butter and olive oil over medium-low heat. As the butter melts, you’ll start to smell that rich, comforting scent filling your kitchen.
3. Add the onions to the skillet and cook, stirring occasionally. You’re aiming for a beautiful light brown color, which will take about 25 minutes. Don’t rush this step; caramelized onions can elevate the dish significantly!
4. Meanwhile, in a large bowl, combine the lean ground beef, egg, dry bread crumbs, milk, salt, pepper, and herbs de Provence. Mix gently to combine, but be careful not to overwork the meat.
5. Shape the mixture into 14 to 16 meatballs, ensuring they are evenly sized for consistent cooking. Place them on the prepared baking sheet.
6. Once the onions are caramelized to your liking, sprinkle the flour over them. Stir to incorporate and cook for an additional 2 minutes. This step will help thicken your sauce.
7. Gradually whisk in the beef broth, bringing it to a boil. The mixture should start to thicken slightly. Add the fresh thyme, along with a sprinkle of salt and pepper to taste.
8. Reduce the heat to low and keep the sauce warm as you finish the meatballs. Bake the meatballs in the oven for 15 to 18 minutes until they are cooked through.
9. When the meatballs are done, gently add them to the cast iron skillet with the caramelized onions. Stir to coat the meatballs in that luscious sauce.
10. While the meatballs are mingling with the sauce, place the baguette slices on the same baking sheet. Stick them under the broiler until they become a little crispy, which should take just a few minutes.
11. Finally, nestle the crispy baguette slices between the meatballs and sprinkle the Gruyere cheese over everything. Place the skillet under the broiler until the cheese is melted and bubbly.
12. Serve hot, garnished with chopped parsley, and perhaps a side of creamy mashed potatoes for a complete meal.
