01 - Preheat oven to 375°F. Lightly grease a 9x13 inch baking dish.
02 - Heat olive oil in a large skillet over medium heat. Sauté leeks, mushrooms, and garlic until softened, about 3 minutes.
03 - Add ground turkey, breaking it apart with a spoon. Cook until no longer pink, approximately 5 minutes.
04 - Stir in cauliflower, butternut squash, and kale. Continue cooking 4 to 5 minutes until vegetables begin to soften.
05 - Add thyme, nutmeg, salt, and pepper. Stir in heavy cream and half of the Gruyère and mozzarella cheeses. Remove from heat.
06 - Transfer mixture to the prepared baking dish. Evenly sprinkle remaining Gruyère, mozzarella, and Parmesan on top.
07 - Bake uncovered for 25 minutes, or until bubbly and golden brown.
08 - Let the casserole rest for 5 minutes before portioning and serving.