Season steak with 1/2 of the salt and pepper. Heat 1 Tbsp (15 mL) olive oil in large skillet over medium-high heat and brown steak in 2 batches; remove from heat and keep warm.
Heat remaining 1 Tbsp (15 mL) olive oil in same skillet over medium-high heat and cook mushrooms and onion, stirring occasionally, until lightly golden, about 8 minutes. Stir in garlic and remaining salt and pepper. Return beef and juices to skillet.
Meanwhile, stir sour cream into hot Pasta, then stir into skillet. Sprinkle, if desired, with chopped fresh parsley.