In a large nonstick skillet, heat 2 tsp. EVOO over medium-high heat. Add the onions and a pinch of salt and cook stirring, until softened, 5 to 7 minutes. Transfer to a plate.
In the same skillet, heat the remaining 1 tbsp. EVOO over medium-high heat. Add the steak and a pinch of salt; cook stirring until just lightly browned, 2 to 3 minutes. Transfer to a bowl and cover loosely with foil to keep warm.
In the same skillet, add the mushrooms and cook, undisturbed, until beginning to brown, 2 to 3 minutes. Sprinkle with a pinch of salt, toss, then cook until the mushrooms are browned, about 2 minutes. Stir in the beef, any juices from the bowl and 1/4 cup water. Cook until heated through, about 1 minutes.
Scoop some of the bread out of the cut sides of the baguette. Layer the bottoms with the steak and mushrom with the juices. Top with the onions and cheese; broil for 1 minutes to melt the cheese. Cover with the baguette tops.