In a small bowl combine the tomato, onion, jalapeno, lime zest, salt and pepper. Toss to combine. Taste for seasoning. Add a squeeze of lime juice for extra flavor.
Spray a 9" skillet with vegetable spray. Heat over medium heat until pan is very hot, but not scorching. Place one flour tortilla in the pan. Top with tuna salad and spread evenly over tortilla, leaving about 1/2" gap from the edge of the tortilla.
Add the avocado slices in a circle. Top with shredded cheese and the 2nd tortilla. Cook until the bottom of the tortilla begins to brown and crisp, 2-3 minutes. Use a large spatula to carefully slip under the quesadilla. Place your hand on top of the quesadilla and in one quick movement, flip the quesadilla in the pan. Cook until the bottom of the quesadilla is crisped and browned and the cheese is melty. Remove from the heat.
Transfer quesadilla to a cutting board and let it rest for a minute before cutting into it. Use a large chef's knife to cut into 4 equal pieces -- not sawing the quesadilla -- but setting the sharp blade across the quesadilla and pressing from the tip to the hilt, until it splits. If anything oozes out, nudge it back in with a spoon (or your finger).