Preheat oven to 450ºF. Tear off 4 10-inch squares of aluminum foil. Brush each square with 2 tsp. olive oil. Place 1 cup spinach on each square, slightly off center. Top with a chicken breast half and some mushrooms. Drizzle 1 tsp. teriyaki sauce over each portion; sprinkle with salt.
Fold each piece of foil over to cover chicken and vegetables. Crimp edges together tightly to seal. Place packets on a rimmed baking sheet and bake until chicken is cooked through, 15 to 20 minutes. Serve hot.