Preheat the oven to 400F. Line a baking tray with parchment paper.
In a medium bowl, combine the tuna, yogurt, dijon mustard, celery, and lemon zest. Divide the tuna among two slices of bread, and top each with a slice of cheese.
Transfer to the baking tray, and bake for 5 to 6 minutes, or until the cheese has melted. Serve and enjoy!