Preheat oven to 375° F.
Place cauliflower in a microwave-safe bowl with a lid. Add ¼ cup water, and microwave for 7-8 minutes, checking occasionally. The cauliflower must be very soft to mash into a smooth consistency.
Place cauliflower in bowl with milk and butter. With a potato masher, mash the cauliflower to desired consistency. For a smoother texture similar to mashed potatoes, add cauliflower to a blender and blend until smooth. Add salt to taste.
Meanwhile, heat oil in a large skillet over medium heat. Add onions, carrot, and celery and cook until softened. Add garlic and cook for 1-2 minutes. Add ground turkey and broth and continue to cook until turkey is cooked through. Add salt and pepper to taste.
When turkey is fully cooked, place mixture into a casserole dish. Top with mashed cauliflower, then sprinkle with shredded cheese.
Place casserole in oven and bake until cheese is melted and bubbly. Enjoy!