Heat a grill or grill pan over medium-high heat until hot; brush the grates with the oil. Bring a large pot of salted water to a boil.
Whisk together the mayonnaise, Parmesan, sour cream, lemon juice, garlic, 1/2 teaspoon salt and a few grinds of pepper in a large mixing bowl.
Season the steak generously with salt and pepper. Grill the steak, turning once, 3 to 4 minutes per side for medium rare. Transfer to a cutting board and let rest for 5 minutes, then cut into 1/2-inch slices.
Add the pasta to the boiling water and cook according to the package directions. Drain and rinse under cold water to cool; add to the mixing bowl.
Add the steak, carrots, celery and onion to the mixing bowl and toss to combine. Season with salt and pepper. Serve sprinkled with more Parmesan and black pepper.
The pasta salad is best if served right away.