First, grab a medium-sized mixing bowl. Start by draining your can of tuna thoroughly. Then, flake the tuna with a fork, breaking it into bite-sized pieces. Toss in the diced celery, chopped carrots, and minced red onions.

In the same bowl, add the mayonnaise, Dijon mustard, red wine vinegar, smoked paprika, and season with salt and black pepper. Mix this all together until well combined. You want every piece of tuna covered in this flavorful mixture.

Next, take your slices of whole wheat bread. You can toast them in a toaster, or for an extra crispy texture, place them on a skillet over medium heat. Use a little oil spray to help achieve that golden color. Toast each side for about 2-3 minutes until light brown.

Now, it’s time to assemble! Place the toasted bread on a baking sheet or oven-safe plate. Layer the tuna mixture generously on top of each slice. Then, add the sliced tomatoes and top with your cheese.

Turn your oven to broil. Pop your baking sheet under the broiler for about 2-4 minutes, keeping a close watch. You want the cheese to melt and bubble but not burn. If you prefer a little browning on top, leave it for a touch longer while checking it regularly.

Once the cheese is perfectly melted, remove from the oven, and let it cool for a minute. Slice it if desired, and serve it hot! Enjoy every cheesy, tuna-filled bite.