Rub steak with 1 tsp. salt and pepper.
In a large, heavy skillet over medium-high, heat 1 tbsp. oil. Cook steak, flipping once, until cooked to desired degree of doneness, 2-3 minutes per side. Transfer to a cutting board.
In same skillet heat remaining 1 tbsp. oil. Add bell pepper, asparagus, and mushrooms and ¼ tsp. salt and cook, stirring frequently, until tender, about 5 minutes.
In a large, dry skillet over medium, heat tortillas 1 at a time, flipping once, until heated through and pliable. Transfer to a plate and warm remaining tortillas, stacking them on top of each other.
Thinly slice steak and serve with vegetables in tortillas. Top with avocado and salsa.