Pour in the heavy cream and whole milk, stirring to combine and scraping up any bits from the pan. Allow the sauce to simmer for 3 to 5 minutes until it thickens slightly.
Gradually stir in the parmesan until the sauce becomes smooth and glossy. Adjust seasoning with salt and pepper and add red pepper flakes if desired.
Add the cooked tortellini and sliced steak to the skillet, tossing gently to coat everything in the sauce, warming through for 1 to 2 minutes.
Plate the pasta and sprinkle with chopped parsley and cracked black pepper before serving immediately.
Notes
Letting the steak rest is crucial for juiciness. Avoid burning the garlic for the best flavor in the sauce.