Preheat the broiler. Spray a large skillet with cooking spray and set over high heat.
Season steaks on both sides with coarse salt and pepper and place in the skillet. Sear 1 per side and then transfer to a broiler pan and broil 3 to 5 minutes for medium rare or to the desired doneness. (Cooking time will depend on the thickness of the steaks.) When done, transfer to a platter and let rest for 15 minutes.
While the steak is broiling, return skillet to stove and reduce heat to medium. Add ½ tablespoon oil and all onions and turn with tongs to coat. Cook, stirring frequently about 10 minutes, or until softened and golden brown. Spray again with cooking spray if pan becomes too dry. Transfer to a platter and keep warm.
Add garlic and remaining ½ tablespoon oil to skillet; cook over medium-high heat about 30 seconds. Add mushrooms and turn to coat. Season with black pepper and cook, stirring frequently for 10 to 15 minutes, or until mushrooms have released their liquid and begin to turn golden brown. Add sour cream and broth and cook, stirring, until no sign of white and the mushrooms are coated with a light sauce. Transfer to a platter and keep warm.
To serve, place 1 steak on each plate, and accompany each with half the onions and mushrooms.