• Preheat your oven to 400 degrees F. Prepare two sheet pans with parchment paper or aluminum foil sprayed with nonstick spray.
• Combine all the meatball ingredients in a large mixing bowl. Mix to combine.
• Form the ground chicken mixture into 20 even-sized meatballs. Place them on the prepared sheet pan.
• Bake the meatballs in the oven for 20 minutes, or until they're just cooked through and slightly brown on the bottom.
• While the meatballs are cooking, place a large stock pot, dutch oven, or high-sided skillet over medium low heat. Add in the olive oil and butter. As the butter starts to melt, add the sliced onions.
• Stir the onions every few minutes, making sure they don't burn on the bottom. Continue this process for about an hour. The onions will first turn translucent, then they will start to slowly break down and turn golden brown.
• While the onions cook on the stove, slice the french bread into 10 1-inch thick slices. Place them on the other sheet pan and toast them in the preheated oven for about 5 minutes, or until they're lightly browned.
• When the onions are soft, brown, and caramelized, stir in the minced garlic. Cook 30 seconds before pouring in the wine and seasoning with thyme, salt, and pepper. Reduce the wine by simmering it until there's barely any left in the pan.
• When the wine is reduced, add the chicken broth. Simmer again and reduce the broth for about 10 minutes, or until the sauce thickens to a creamy brown consistency.
• Add the baked chicken meatballs to the french onion sauce and stir gently to coat them fully in the sauce.
• Top each meatball with freshly shredded gruyere cheese and cover the pan with a lid to melt the cheese for about 3 minutes.
• To serve, garnish with chopped fresh parsley and scoop about 2 cheesy meatballs onto each of the slices of toasted bread. Spoon extra sauce and caramelized onions over top.

Storage

• To store: Allow the meatballs to cool completely before placing them, and the rest of the onions and sauce, in an airtight container. Store them in the refrigerator up to 4 days.

  Store the bread separately on your counter or in your pantry up to 3 days. Slice it and toast it just before serving.

  To freeze: Allow the meatballs to cool completely before placing them, and the rest of the onions and sauce, in an airtight container. Store them in the freezer up to 4 months for best quality.

  To reheat: Place the meatballs in the refrigerator and allow them to defrost overnight. When ready to eat, scoop meatballs, onions, and sauce into a large skillet over medium heat.

  Stir meatballs constantly so they don't stick or burn and heat them until they're just warmed through in the middle. Serve the meatballs on freshly sliced and toasted bread.