Start by preheating your oven to 400°F (200°C). Line a baking sheet with parchment paper so the quesadillas don’t stick while baking. This step also helps them crisp up nicely without extra oil.
Heat olive oil in a large skillet over medium heat. Add chopped onion and minced garlic, cooking until soft and fragrant. This step builds the base flavor for the filling and makes the quesadillas more savory.
Add sliced mushrooms to the pan along with cumin, chili powder, salt, and pepper. Cook until the mushrooms release their liquid and become tender. Let the liquid evaporate fully, as this prevents soggy quesadillas.
Stir in the chopped spinach and cook just until wilted. Fresh spinach will reduce quickly, leaving a flavorful mixture ready to be layered into tortillas. Remove the pan from heat once done.
Place four tortillas on the prepared baking sheet. Evenly spread the spinach and mushroom filling over them, then sprinkle with shredded mozzarella cheese. Cover each with another tortilla to form a sandwich.
Lightly spray the top of the tortillas with cooking spray. Bake in the oven for 8–10 minutes, or until the tortillas turn golden brown and the cheese inside is fully melted.
Remove from the oven and let the quesadillas rest for a minute before slicing. Use a sharp knife or pizza cutter to cut them into wedges. Serve hot with your favorite dips like salsa, guacamole, or sour cream.