Preheat oven to 350°F. Butter a 8x8" baking dish and set aside.
Cook pasta al dente, according to package directions. Drain, reserving 1/2 cup pasta water.
In a skillet, melt the butter over medium-heat. Add the onions and cook 3-5 minutes. Then add the garlic and cook an additional minute. Next add in the turkey, cream of mushroom soup and sour cream. Add in pasta water if need to thin out. Salt and pepper to taste.
Toss sauce with noodles and pour into baking dish. Top with cheese and bake for 25-30 minutes, or until the tops is bubbly and lightly golden.
Optional: Add in 1/2 cup sliced mushrooms with the onions. You can substitute 1/2 teaspoon onion powder and 1/2 teaspoon garlic powder if you'd rather omit the onions and garlic. Simply mix the seasonings with the melted butter and add to the cream mixture.