French Onion Chicken Meatballs

Preheat oven to 425°. Line a large baking sheet with foil and rub with oil.  Make meatballs: In a large bowl, combine ground chicken, Gruyère, bread crumbs, parsley, egg, and garlic. Season with salt and pepper. Form into 16 meatballs, then place on prepared baking sheet and bake until golden and cooked through, 25 minutes. Meanwhile make sauce: In a large skillet over medium heat, melt butter. Add onions and cook until very soft and golden, 25 minutes, stirring often. Add garlic and cook until fragrant, 1 minute more. Add broth and thyme and season with salt and pepper. Bring to a boil, then reduce heat and let simmer until slightly thickened, 10 minutes.  Add meatballs to skillet and sprinkle with Gruyère. Cover and cook until meatballs are warmed through and cheese is melty, 5 minutes. Serve warm, garnished with thyme.