1. Preheat your oven to 375°F (190°C). Grease a baking dish with butter to prevent sticking.
2. In a large skillet, heat olive oil over medium heat. Add the chicken breasts and cook until browned on both sides, about 5 minutes per side. Remove and set aside.
3. In the same skillet, add onions and garlic, sautéing until fragrant and translucent, about 3 minutes.
4. Add mushrooms to the skillet and cook until they release their moisture and begin to brown, roughly 5 minutes.
5. Stir in the spinach, cooking until just wilted. Season with salt, pepper, and dried thyme.
6. Pour in the chicken broth and heavy cream, bringing the mixture to a simmer. Let it cook for about 5 minutes until slightly thickened.
7. Return the chicken breasts to the skillet, spooning the sauce over the top.
8. Transfer everything to the prepared baking dish. Sprinkle with Parmesan cheese and breadcrumbs if using.
9. Bake in the preheated oven for 25-30 minutes, or until the cheese is golden and bubbly, and the chicken is cooked through.
10. Let it rest for a few minutes before serving to allow the flavors to meld together beautifully.

One of my little secrets, learned from Mama Lou, is to let the dish rest after baking. This not only enhances the flavors but also makes serving easier and more beautiful.