1. In a large skillet, heat 1 tablespoon oil over medium. Add mushrooms; season with salt and pepper. Cook, stirring occasionally, until browned, 8 to 10 minutes. Transfer to a plate (reserve skillet).

2. Heat remaining tablespoon oil in skillet. Add as much spinach as will fit; season with salt and pepper. Toss spinach, adding more to skillet as room becomes available. Cook until tender, about 2 minutes. Remove from heat, drain off any excess liquid, and stir in vinegar. Top spinach with mushrooms.