Place the squash in a large microwave-safe dish with 1/2 cup of water. Cover the bowl and cook the squash in the microwave for 12 to 14 minutes, until the squash is soft.
Meanwhile, bring a medium skillet sprayed with nonstick spray to medium heat on the stove. Add onion and, stirring occasionally, cook until it's slightly browned, 2 to 3 minutes.
Raise the heat to medium high. Add turkey, chopped garlic, 1/4 teaspoon of salt, garlic powder, onion powder, and black pepper to the skillet. Cook and crumble until the turkey is fully cooked, about 12 minutes.
Drain any excess liquid from the skillet, and transfer the mixture to a casserole dish sprayed with nonstick spray. Evenly top with gravy, followed by frozen mixed veggies. Set aside.
Drain the excess water from the squash and mash well. Add butter and the remaining 1/4 teaspoon of salt and mix thoroughly. Evenly layer the squash mixture over the mixed veggies.
Bake in the oven until the topping is slightly crispy, 20 to 25 minutes. Enjoy!