Prepare the Tortellini:

Cook the tortellini according to package instructions. Drain and set aside.

Cook the Beef:

In a large skillet, heat olive oil over medium-high heat. Add the beef pieces and season with salt and pepper. Brown the beef on all sides, then remove it from the skillet and set aside.

Sauté the Vegetables:

In the same skillet, add the chopped onion and minced garlic. Sauté until the onion is translucent. Add the sliced mushrooms and thyme. Cook until the mushrooms are soft and browned.

Make the Sauce:

Return the beef to the skillet and pour in the beef broth. Simmer for about 15-20 minutes, or until the beef is tender. Stir in the heavy cream or sour cream, and let it simmer for an additional 5 minutes.

Combine and Serve:

Add the cooked tortellini to the skillet and gently mix to combine all ingredients. Adjust seasoning with salt and pepper, if needed. Serve hot, garnishing with chopped parsley and grated Parmesan cheese, if desired.