Prepare the Meatballs: Begin by preheating the oven to 450°F. Grease a 9x13-inch baking dish with olive oil. In a separate bowl, lightly dampen the bread under water, then squeeze out any excess moisture. Crumble the softened bread into small pieces. Combine the bread, ground chicken, beaten egg, and a pinch each of salt and pepper. Using olive oil-coated hands, shape the mixture into meatballs, roughly the size of two tablespoons, and arrange them in the prepared baking dish. Bake for 15 minutes or until the meatballs are crisp on the outside but not fully cooked through.

Caramelize the Onions: While the meatballs bake, melt the butter in a large oven-safe skillet over medium-high heat. Add the thinly sliced onions and cook, stirring occasionally, for about 10 minutes until softened. Slowly add the white wine, ¼ cup at a time, allowing each addition to be absorbed into the onions before adding more. Continue cooking for another 10-15 minutes, ensuring the onions caramelize deeply.

Prepare the Broth: To the caramelized onions, add the chopped garlic, sliced mushrooms, fresh thyme, and sage. Season with salt and pepper, and cook for an additional 3-4 minutes. Pour in the broth, bring to a simmer, and then lower the heat to maintain a gentle boil. Add the pre-baked meatballs to the skillet, simmering them in the broth for approximately 10 minutes, or until fully cooked through. Stir in the cream for extra richness.

Toast the Bread and Broil: Arrange the French bread slices on a baking sheet and toast them in the oven for about 10 minutes, until they are completely dry. Switch the oven to broil, then place the toasted bread pieces over and around the meatballs. Top with shredded gruyere cheese and broil for 3-5 minutes, or until the cheese is bubbly and golden brown. Finish with a sprinkle of fresh thyme and serve immediately.