These chicken meatballs are flavorful, sweet, and juicy. You can also use leftover French onion soup to make the sauce base. These are made with chicken but ground pork makes an even richer variation.
Preheat the oven to 425°F and line a rimmed baking pan with foil. Add a tablespoon of oil to the pan and evenly coat the surface, set aside.
Combine all meatball ingredients in a bowl and mix until evenly combined. Roll into 16 meatballs and place on the reserved baking sheet. Bake at 425ºF for 25 minutes, or until golden and cooked through.
While meatballs cook, prepare the onions. Heat a large oven-safe pan over medium high heat and add butter, onions, and a pinch of salt. Cook until soft and golden, about 30 minutes, stirring often, especially as they become golden. If they look too brown at any point, add a splash of water. When golden, add the garlic and thyme and cook until fragrant, about 1 minute. Add the broth and bring the mixture to a boil. Reduce heat and simmer for about 10 minutes, or until thickened. Taste and season with salt and pepper.
Add the meatballs to pan and sprinkle with Gruyère. Broil or cook on high for 5 minutes, or until cheese just begins to brown.