Step 1
Preheat the oven to 400 degrees F (200 degrees C).

Step 2
Mix ground chicken, bread crumbs, egg, onion soup mix, and garlic together in a medium bowl until the ingredients are well incorporated. Roll into sixteen 1 1/2-inch meatballs.

Step 3
Melt butter in a saucepan over medium heat. Add onions and sugar; saute until onions are softened, about 12 minutes.

Step 4
Meanwhile, heat olive oil in an oven-safe skillet over medium-high heat. Add meatballs and cook, flipping occasionally, until edges are browned, about 4 minutes.

Step 5
Transfer skillet to the preheated oven and bake until meatballs are no longer pink in the center, about 15 minutes. Check the meatballs halfway through and flip if they are browning too much on the bottom.

Step 6
Add red wine, salt, and pepper to the onions and stir. Cook over medium heat for 3 minutes. Stir in beef broth and thyme. Bring to a boil. Reduce heat to low and slowly stir in flour. Cook until slightly thickened, 2 to 3 minutes more.

Step 7
Remove meatballs from the oven and pour onion sauce on top. Cover evenly with Gruyere cheese and return to the oven until melted, about 5 minutes.