01 - Preheat oven to 400°F. 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.
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 and sauté for 3-4 minutes until softened and browned. Add minced garlic and cook for 30 seconds until fragrant. Add spinach and toss until just wilted, about 1 minute. Remove from heat.
05 - Arrange seared chicken breasts in the prepared baking dish. Spoon the mushroom-spinach mixture evenly over the chicken. Pour heavy cream over the entire dish. Sprinkle shredded mozzarella cheese on top.
06 - Bake uncovered for 25-30 minutes until chicken reaches an internal temperature of 165°F and cheese is golden and bubbly.
07 - Let the casserole rest for 5 minutes before serving to allow juices to redistribute.