Slice the Roma tomatoes in half long-ways so you have 2 long ovals. Gently scoop out as many seeds as you can, leaving the flesh. Season to taste with salt and pepper. If you have time, allow the tomatoes to sit on the counter for 15-20 minutes. The salt will draw out excess moisture, and you can pour this off before proceeding. If you don’t have time, the finished tomatoes will have a bit more water content but will still be delicious.
In a small bowl, combine the tuna, mayonnaise, celery, onion, lemon juice, dill and a few grinds of the pepper mill. The tuna salad may be prepared several hours in advance, covered and refrigerated.
Preheat the broiler to high. If the tomatoes have been sitting, pour off any moisture that has collected inside. Stuff each tomato half with one quarter of the tuna salad. Arrange the cheese on top; if using sliced cheese, break to fit, allowing it to extend a few millimeters over the edge. (Depending on tomato proportions, you may not need four full slices.) Arrange the tomatoes on a baking sheet, and broil the tomatoes until the cheese is melted with a few golden brown spots on the top, about 1 to 2 minutes. Make certain to watch very closely after the first minute so as not to burn the cheese.** Remove from the oven and enjoy immediately. For a pretty look, you may wish to garnish the plate with fresh herbs, if available.
Cool, cover and refrigerate any uneaten melts. Served cold, the leftovers make a welcome lunch.