Place the mozzarella, crumbled feta, chopped spinach, diced tomato, thinly sliced red onion, and a pinch of black pepper into a medium bowl. Stir until everything is evenly combined.

Lay one tortilla flat on your work surface. Spoon about one fourth of the filling onto one half of the tortilla. Fold the empty half over the filling to make a semicircle.

Heat a nonstick skillet over medium heat. Add the tablespoon of olive oil and swirl to coat the surface.

Place the folded tortilla in the skillet. Cook undisturbed for 3 to 4 minutes until the bottom is golden brown and crisp.

Flip the quesadilla and cook the second side for another 3 to 4 minutes until golden and the cheeses are melted through.

Transfer the cooked quesadilla to a cutting board and let it rest for 1 minute to set. Slice into wedges and serve warm.