Prepare egg noodles for one minute less than instructed on the package. Drain and set aside.
Preheat oven for 375 degrees.
In a large pot, brown the ground beef and onion together. Season with salt and pepper.
Add the minced garlic and cook for about minute, until fragrant. Remove beef from pan and set aside.
In the same pan you browned the beef, add the butter to the pan and swirl around until melted.
Whisk in the flour and cook for 1 minute, stirring continuously.
Add in half of the beef broth and whisk until incorporated, then add the second half.
Mix in the sour cream, mustard, and 3/4 cup of the cheese until well-combined. (Sour cream will keep melting and will eventually incorporate into the mix.) Allow mixture to simmer for 3-5 minutes, until thickened. Add the ground beef back to the sauce.
Pour into a 9×13 dish and top with remaining 1 cup of cheese.
Bake uncovered at 375 degrees for 15-20 minutes, until the cheese is hot and melted. Serves 4-5.
