01 - Preheat oven to 400°F. Lightly grease a 9x13-inch baking dish with cooking spray or a small amount of olive oil.
02 - Pat chicken breasts dry with paper towels. Season both sides generously with salt and black pepper, pressing the seasoning into the meat to adhere.
03 - Heat olive oil in a large skillet over medium-high heat until shimmering. Add chicken breasts and sear for 2-3 minutes per side until golden brown. Transfer to a plate and set aside.
04 - In the same skillet, add sliced mushrooms. Sauté for 3-4 minutes until they release moisture and begin to brown slightly.
05 - Add minced garlic to the mushrooms and cook for 30 seconds until fragrant. Add fresh spinach and toss until just wilted, about 1 minute. Remove from heat.
06 - Arrange seared chicken breasts in the prepared baking dish. Top evenly with the mushroom-spinach mixture, distributing it across all pieces.
07 - Pour heavy cream evenly over the chicken and vegetables. Sprinkle shredded mozzarella cheese generously across the top, covering the surface.
08 - Bake uncovered for 25-30 minutes until chicken reaches an internal temperature of 165°F and cheese is melted and golden brown with bubbly edges.
09 - Remove from oven and let the casserole rest for 5 minutes. This allows the juices to redistribute and the sauce to thicken slightly for easier serving.