Step 1: Heat olive oil in a large oven-safe skillet or Dutch oven over medium-high heat. Add ground beef and cook, breaking it up with a spoon, until browned. If using sirloin, sear quickly until browned on all sides. Drain excess fat and remove beef from skillet; set aside.
Step 2: Add chopped onion to the same skillet and cook until softened, about 3-5 minutes. Add sliced mushrooms and cook until they release their liquid and begin to brown, about 5-7 minutes. Stir in minced garlic and cook for 1 minute more until fragrant.
Step 3: Sprinkle flour over the vegetables and stir continuously for 1 minute to cook out the raw flour taste. Gradually whisk in the beef broth and milk, stirring constantly to prevent lumps. Bring the mixture to a simmer.
Step 4: Stir in the Worcestershire sauce and Dijon mustard. Return the cooked beef to the skillet. Add the tortellini to the simmering sauce, ensuring it's mostly submerged. Cover the skillet and cook for 8-10 minutes, or until the tortellini is al dente, stirring occasionally.
Step 5: Remove the skillet from the heat. Stir in the sour cream until fully incorporated and the sauce is smooth and creamy. Do not bring back to a boil after adding sour cream, as it may curdle.
Step 6: Taste and season with salt and freshly ground black pepper as needed. Garnish with fresh chopped parsley before serving hot.