Heat a cast iron pan that has grill marks to medium heat.
While the pan is heating up, drain your can of tuna very well.
To make the tuna mixture: combine the tuna, mayo, dijon mustard, onion, celery, salt and pepper.
Take your tomato slices and press them between two pieces of paper towel to get as much of the moisture out as you can (if you skip this step you’ll be left with a very soggy wrap!)
Assemble your wrap by first adding the spinach and then the tuna.
Now, take your slice of cheese and cut it in half then place the two halves end-to-end down the length of the wrap.
Place your 3 tomato slices on top.
Spray your hot pan with non-stick spray, roll up your wrap and place it seam side down on the hot pan.
Cover your wrap on the pan with a lid to help lock in the heat.
Cook for 3 minutes (check for grill marks before flipping! You want those to be visible- how satisfying!), flip, and cook for another 3 minutes.
Cut your wrap in half with a serrated knife and enjoy!