Marinate the steak overnight in your preferred marinade if desired.
Bring a large pot of salted water to a boil and cook the tortellini according to package directions; drain and set aside to cool slightly.
Preheat a grill or grill pan to medium-high heat. Season the sirloin and grill to your desired doneness, about 4–6 minutes per side for medium depending on thickness.
Let the steak rest 5 minutes, then slice into bite-sized pieces.
In a bowl, whisk together buttermilk, mayonnaise, sour cream, Parmesan, and pesto until smooth to make the dressing.
Toss the chopped romaine with cooked tortellini in a large bowl. Top with sliced steak and drizzle the dressing over everything; toss gently to combine.
Serve immediately.
