Preheat your oven to 400°F and line a large baking sheet with parchment paper.
Slice the sweet potato into 1/2-inch thick rounds and arrange them in a single layer on the prepared baking sheet.
Roast the sweet potato rounds for 20 minutes, flipping them halfway through, until they are tender and slightly caramelized.
While the potatoes roast, drain the tuna and place it in a medium bowl.
Add the Greek yogurt, finely diced celery, lemon juice, sea salt, black pepper, garlic powder, and dried dill to the tuna, mixing until well combined.
Remove the sweet potatoes from the oven and spoon a generous portion of the tuna mixture onto each round.
Top each melt with a sprinkle of shredded sharp cheddar cheese.
Return the pan to the oven for 3-5 minutes, or until the cheese is melted and bubbly.
Garnish each melt with a fresh slice of avocado and serve immediately.