In a large skillet, heat the olive oil over medium heat. Add the diced onion and sauté until translucent, about 5 minutes.
Stir in the minced garlic and cook for an additional minute until fragrant.
Add the chicken thighs to the skillet, browning them on all sides (about 5-7 minutes). Remove from heat and set aside.
In the same skillet, add the sliced mushrooms, carrots, and zucchini. Sauté for about 5-7 minutes until the vegetables are tender.
Pour in the chicken broth and heavy cream, then add the dried thyme, rosemary, salt, and pepper. Stir until well combined.
In a large casserole dish, layer the sautéed vegetables and chicken thighs evenly. Top with fresh spinach and sprinkle the grated Gruyère cheese over the entire mixture.
Cover the dish with aluminum foil and bake in the preheated oven for 30 minutes.
After 30 minutes, remove the foil and bake for an additional 15 minutes, or until the cheese is golden and bubbly.
Remove from the oven and let the casserole cool for a few minutes before serving.
Notes
Serve the casserole directly from the dish with fresh parsley sprinkled on top for added color and a warming piece of crusty bread on the side.