Crispy Mushroom, Spinach and Avocado Quesadillas

Ingredients (10)

• 1 tablespoon olive oil
• 1/2 red onion, chopped
• salt
• 8 to 10 ounces cremini mushrooms (AKA baby bellas), thinly sliced
• 6 ounces baby spinach, roughly chopped
• 1/2 small lime, juiced
• 2 1/2 cups shredded raw Jack or cheddar cheese, divided
• 2 avocados from Mexico, cut into thin slices
• 4 medium whole wheat tortillas
• jarred red salsa