Preheat your oven to 425°F (220°C). Line a baking sheet with parchment paper or lightly grease it with cooking spray.

Heat 2 tablespoons of olive oil in a large skillet over medium heat. Add the sliced onion and sauté about 4–5 minutes until soft and lightly caramelized.

Toss in the sliced mushrooms and cook until they release their liquid and begin to brown, about 5–7 minutes. Stir in the minced garlic and cook 30–60 seconds more until fragrant.

Add the fresh spinach in batches, stirring until it fully wilts. This should take 2–3 minutes. Season with salt, pepper, cumin, and smoked paprika if using, then remove from heat and let cool slightly.

In a medium bowl, combine the shredded Monterey Jack and cheddar cheese. If you like a creamier filling, stir in the cream cheese or Greek yogurt now. Then fold in the cooked spinach-mushroom mixture until evenly coated.

Lay out 4 tortillas on a flat surface. Divide the filling evenly among them, spreading it over half of each tortilla. Top each with another tortilla to make a sandwich.

Lightly brush the top of each quesadilla with melted butter or spray with cooking spray.

Place them on the prepared baking sheet and bake for 8–12 minutes, flipping halfway through, until both sides are golden brown and the cheese is melted.

Remove quesadillas from the oven and let them rest for 2–3 minutes. Slice into wedges using a pizza cutter or sharp knife. Serve hot with salsa, sour cream, or avocado slices.

If you prefer an extra-crisp edge, pop the baked wedges into a hot skillet for 30–60 seconds per side before serving.