Preheat the oven to 400 degrees. Place the potato slabs down on a parchment paper lined baking sheet. Brush both sides with olive oil and season both sides with salt, pepper and garlic powder. Roast for 30 minutes, flipping halfway through.

Meanwhile, prepare the tuna salad. Place the tuna, avocado, and Dijon mustard in a medium mixing bowl and mash together and mix until combined. Season with pepper and add the onions and celery and mix well.

Slice the tomato in 1/8” thick slices crosswise. You need eight total slices.

When potatoes are done cooking, top each with the tuna salad, dividing evenly. Top the tuna salad with two tomato slices. Then, top with either one cheese slice in the center of the sandwich or use two overlapping cheese slices (depending on how cheesy you want it! I like less cheese, but a traditional tuna melt is nice and cheesey.) Place back into the oven and let cook until cheese melts, about 5 minutes. Once done, remove from oven and immediately garnish with chives and more pepper. Serve warm.