Marinate the steak overnight in your preferred marinade, covering it and refrigerating until ready to cook.
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 over medium-high heat. Grill the marinated sirloin to your desired doneness, then rest a few minutes and slice into bite-sized pieces.
In a small bowl, whisk together the buttermilk, mayonnaise, sour cream, Parmesan, and pesto until smooth to make the dressing; taste and adjust pesto as needed.
In a large bowl, combine the chopped romaine and cooked tortellini. Add the sliced steak on top, then drizzle with the dressing and toss gently to combine.
Serve immediately and enjoy.