Marinate the steak overnight in your preferred marinade, or at least 30 minutes if short on time.
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. Grill the marinated top sirloin to your desired doneness, about 4–6 minutes per side depending on thickness.
Transfer the steak to a cutting board and let it rest 5 minutes, then slice into small pieces or bite-sized strips.
In a mixing bowl, whisk together the buttermilk, mayonnaise, sour cream, Parmesan, and pesto until smooth and combined.
Toss the chopped romaine and cooked tortellini in a large bowl, drizzle with the dressing, and toss to coat evenly.
Top the salad with the sliced steak, adjust seasoning if needed, and serve immediately.