Into the same skillet add 8 oz of sliced mushrooms, season with salt and pepper to taste and saute for 3 -5 minutes, stirring few times. Transfer sauteed mushrooms & onion to a plate, cover with a lid to keep warm.
Into the same skillet add 1 tablespoon of oil and preheat to high medium/heat. Once it’s hot, working in 2 batches add steak and season with 1/2 of Montreal Steak Seasoning and salt and pepper to taste.
Cook on high heat without stirring for couple minutes, then flip on the other side and saute until its browned, about couple more minutes. Transfer to a plate, cover to keep warm. Repeat this step with the second batch of steak.