Melt the butter in a large saute pan over medium-high heat. Add the turkey and onion to the pan. Cook, breaking up the turkey a bit with a spatula, until no longer pink, about 7 minutes.
Add the mushrooms and cook, stirring occasionally, for about 1 minute. Stir in the garlic powder, 1/2 tsp salt, and 1/2 tsp pepper until well combined.
Stir in the flour until well combined then pour in the chicken broth. Scrape the bottom of the pan with a spatula to remove any stuck on bits and bring the mixture to a boil.
Reduce heat to low and simmer, stirring occasionally, while you cook the noodles. Cook the egg noodles according to package directions.
Stir the sour cream into the sauce and cook until heated through. Season with salt & pepper to taste then serve the sauce over the egg noodles.