Preheat your oven or toaster oven to 350 F (I used a toaster oven).
Open up the English muffin and put both halves on a tray (I used the toaster oven tray). Spoon the tuna salad on the bottom half of the English muffin. Pull apart the string cheese and place the pieces of string cheese on top of the tuna. Place the sandwich in the oven for 10-15 minutes until the cheese melts completely. Remove from the oven, add the slice of tomato and the top half of the muffin and enjoy!