In a large skillet, heat the olive oil over medium heat. Once shimmering, add the finely chopped red onion and sauté for 2-3 minutes, or until the onions become translucent and fragrant.
Incorporate the sliced mushrooms and the minced garlic into the skillet. Cook for about 5-7 minutes, stirring occasionally, until the mushrooms have released their moisture and turn a beautiful golden brown.
Add the chopped spinach and sprinkle the cumin into the mixture. Cook for an additional 2 minutes, or until the spinach wilts down. Season with salt and pepper to taste, then remove the skillet from the heat and set the filling aside.
Heat a separate skillet over medium heat. Place one tortilla in the skillet and sprinkle half of the shredded mozzarella cheese over one half of the tortilla, creating a cheesy base.
Spoon a generous helping of the spinach and mushroom filling on top of the mozzarella. Sprinkle crumbled feta cheese over the filling and finish with the remaining mozzarella.
Carefully fold the tortilla in half. Cook for about 3-4 minutes, allowing the cheese to start melting and the bottom side to turn a delightful golden brown. Gently flip the quesadilla and cook for another 3-4 minutes until the second side is equally golden and crispy.
Once cooked, remove the quesadilla from the skillet and let it cool for a minute before slicing it into wedges. Repeat this process for the remaining tortillas and filling, adjusting the heat if necessary to avoid burning.
Notes
Serve with fresh cilantro and a side of salsa or guacamole.