Heat a large skillet over medium-high heat. Add the olive oil and brown the mushrooms. Remove to a bowl and set aside.
Add the ground beef and brown. Discard all but 1 teaspoon of fat. Bring back to medium high heat and add the noodles, milk, beef stock, mushrooms, cornstarch, salt, pepper and paprika. Stir to combine and cover. Lower the heat to medium and allow to simmer for 10-12 minutes, or until the noodles are cooked through.
Add the sour cream and simmer for 5 more minutes. Serve immediately.
Notes
Tips for Storing
Refrigerator. Store leftovers in an airtight container for up to 3 days. The noodles will keep absorbing the sauce as it sits, so expect a thicker consistency when you reheat.
Reheating. Warm gently on the stovetop over low heat and add a splash of beef broth or milk to loosen the sauce back up. Stir slowly so the noodles stay intact.
Freezer. The beef and sauce portion freeze well for up to 2 months. Skip freezing the noodles and cook a fresh batch when you are ready to eat.
Make-ahead tip. If you know you are planning to reheat this, undercook the noodles by 1 to 2 minutes so they hold their texture once warmed back up.