Drain the tuna thoroughly so the quesadilla crisps instead of steaming.
Place a large nonstick skillet over medium heat. Lightly oil your pan if it isn’t nonstick. Lay the tortilla in the skillet. Sprinkle half the mozzarella over one side, add the tuna in an even layer, then top with the remaining cheese. Fold the tortilla over to make a half-moon.
Press gently with a spatula and cook 2–3 minutes per side, until the tortilla is golden brown and the cheese is melty. If needed, cover the pan for the last minute to help the cheese melt.
Let sit 1 minute, then transfer to a cutting board and slice into wedges.
Notes
Optional (tasty, but not required): a pinch of black pepper, red pepper flakes, dried oregano, or a squeeze of lemon. If you like a creamier melt, stir the tuna with 1–2 teaspoons plain Greek yogurt, mayo, or mashed avocado before assembling.