Melt 2 Tablespoons of butter in a dutch oven or stock pot set over medium heat. Add the sliced mushrooms and sauté for 5-6 minutes. Stir occasionally until the liquids have cooked off and the mushrooms are sizzling. Then transfer to a bowl and set aside.
Melt the remaining tablespoon of butter in the same pot, add the diced onion and cook for 4-5 minutes until softened. Add the garlic and cook for another 30 seconds.
Add the ground beef and break it up with a wooden spoon, then cook for 5 minutes until cooked through. Add the flour and cook for 1-2 minutes, stirring frequently.
Add ½ cup of beef stock and scrape up any brown bits from the bottom of the pan. Add the mushrooms back to the pot, along with the remaining beef stock and Worcestershire sauce, and bring to a simmer. Cook for 2-3 minutes until the sauce is slightly thickened, then stir in the egg noodles and cook for 10 minutes, stirring occasionally, until the noodles are cooked through.
Remove from heat and let cool for 2-3 minutes, then stir in the sour cream and serve.