In a mixing bowl, stir together the mozzarella, feta, chopped spinach, diced tomato, sliced red onion, and a few grinds of black pepper until evenly mixed. Taste and adjust pepper as needed.

Lay one tortilla flat. Spoon one-fourth of the filling onto half of the tortilla. Fold the empty half over the filling and press gently to compact.

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

Place the folded tortilla in the skillet and cook for about 3 to 4 minutes, until the bottom is golden brown and you hear the cheese starting to give.

Flip the quesadilla and cook another 3 to 4 minutes until both sides are crispy and the cheeses are fully melted. Lower the heat if the exterior browns too fast.

Transfer to a cutting board, let rest for a minute to stabilize the filling, then slice into wedges and serve warm.