Cook the egg noodles according to package directions in salted boiling water. Start them when you begin browning the beef so everything finishes at the same time. Drain when tender and toss with a small pat of butter to prevent sticking. Set aside.

Heat olive oil in a 12-inch skillet or 5-quart Dutch oven over medium-high heat. Add ground beef and chopped onion, breaking the meat into small crumbles as it browns. Cook for 5-7 minutes until no pink remains and onions turn translucent. If more than 2 tablespoons of fat remains, drain it off.

Add sliced mushrooms and minced garlic directly to the beef. Cook for 3-5 minutes, stirring occasionally, until mushrooms release their moisture and edges turn golden-brown.

Lower heat to medium and add butter, letting it melt completely. Sprinkle flour evenly over everything and stir constantly for about 1 minute. The mixture should smell nutty and turn light tan as the roux forms.

Slowly pour in beef broth while whisking continuously to prevent lumps. Bring to a gentle simmer and cook for 3-4 minutes, stirring frequently, until the sauce thickens enough to coat the back of a spoon.

Remove the skillet completely from heat. Stir in sour cream and Dijon mustard until the sauce turns smooth and creamy. Season generously with salt and freshly ground black pepper. Removing from heat first prevents the sour cream from curdling.

Return skillet to the lowest heat setting just to warm everything through, about 1-2 minutes. Do not let this boil or the sour cream will break. Serve immediately over the cooked egg noodles and garnish with fresh chopped parsley.