In a bowl, mix mozzarella, feta, chopped spinach, diced tomato, thinly sliced red onion, and a pinch of black pepper until evenly distributed.

Lay one tortilla flat and place one-fourth of the filling on half of it. Fold the empty half over and press lightly to seal.

Warm a non-stick skillet over medium heat and add the olive oil, tilting the pan to coat the surface.

Place the folded tortilla in the skillet. Cook for 3 to 4 minutes until the underside is golden and the cheese begins to melt.

Carefully flip the quesadilla and cook another 3 to 4 minutes until both sides are crisp and the cheese is fully melted.

Remove from the skillet, let rest for about a minute, then slice into wedges and serve warm.