01 - Preheat your oven to 375°F (190°C). Lightly grease a 9x13-inch baking dish.
02 - Season the chicken breasts generously with salt and pepper. Heat 1 tablespoon of olive oil in a large skillet over medium-high heat. Sear the chicken breasts for 2-3 minutes per side until they achieve a light golden color. Transfer the partially seared chicken to the prepared baking dish.
03 - In the same skillet used for the chicken, add the finely chopped yellow onion and sauté for approximately 2 minutes until softened. Add the minced garlic and cook for an additional 30 seconds until fragrant. Stir in the fresh baby spinach and cook just until wilted, about 2 minutes. Remove from heat.
04 - In a medium bowl, whisk together the heavy cream, softened cream cheese, chicken broth, dried thyme, dried basil, and ground nutmeg until a smooth, homogenous sauce forms.
05 - Evenly spread the sautéed spinach and onion mixture over the seared chicken breasts in the baking dish. Pour the prepared cream sauce uniformly over the chicken and vegetable layer.
06 - Evenly sprinkle the shredded mozzarella and grated Parmesan cheeses over the entire surface of the dish. If using breadcrumbs, combine them with the remaining 1 tablespoon of olive oil and sprinkle them over the cheese for a crispy topping.
07 - Bake in the preheated oven for 25-30 minutes, or until the chicken is cooked through (internal temperature reaching 165°F/74°C) and the topping is beautifully golden and bubbling.
08 - Allow the bake to rest for 5 minutes before portioning and serving.