Preheat your oven to 375°F (190°C) and prepare a 9x13 inch baking dish by lightly greasing it with olive oil or cooking spray.

Season chicken breasts on both sides with salt, pepper, and half of the Italian seasoning, ensuring even coverage for maximum flavor.

Heat olive oil in a large skillet over medium-high heat until shimmering, then add chicken breasts and sear for 3-4 minutes per side until golden brown but not fully cooked through.

Remove chicken from skillet and set aside on a plate, leaving the flavorful drippings in the pan for building the sauce.

In the same skillet, add sliced mushrooms and cook for 5-6 minutes until they release their moisture and begin to brown, stirring occasionally.

Add minced garlic to the mushrooms and cook for 1 minute until fragrant, stirring constantly to prevent burning.

Pour heavy cream into the skillet with mushrooms and garlic, then add remaining Italian seasoning and red pepper flakes if using, stirring to combine.

Bring the cream mixture to a gentle simmer and cook for 3-4 minutes until it begins to thicken slightly, stirring occasionally.

Add fresh spinach leaves to the cream sauce and stir until wilted and incorporated, which should take about 2 minutes.

Remove skillet from heat and stir in half of the mozzarella and half of the parmesan cheese until melted and smooth.

Pour half of the creamy sauce mixture into the prepared baking dish, spreading it evenly across the bottom.

Arrange the seared chicken breasts on top of the sauce in a single layer, spacing them evenly in the dish.

Pour the remaining sauce over and around the chicken breasts, ensuring they're well covered with the creamy mushroom spinach mixture.

Sprinkle the remaining mozzarella and parmesan cheese over the top of the entire dish for a golden, bubbly finish.

Cover the baking dish with aluminum foil and bake for 20 minutes to allow the chicken to cook through and flavors to meld.

Remove foil and continue baking for an additional 10-15 minutes until the cheese is golden brown and bubbly and the chicken reaches an internal temperature of 165°F.

Remove from oven and let rest for 5 minutes before serving to allow the sauce to thicken slightly and the chicken to reabsorb its juices.

Garnish with fresh herbs if desired and serve hot with your choice of low-carb sides.