Preheat oven to 400 F and line a baking sheet with parchment paper.
Peel and slice large yellow onion (1) into 3/4" rings.
Grab 2 outer onion rings at a time, and place them on the prepared baking sheet -- you should be able to get at least 9 sets of rings before they start to get too small.
Combine tuna (7 1/2 Ounces), mayonnaise (6 tablespoons), grated parmesan (2 tablespoons), finely chopped celery (1/4 cup), finely chopped pickles (6 tablespoons), lemon juice (1 tablespoon), curry powder (1/2 tablespoon), chopped dill (1 1/2 teaspoon), and garlic powder (1 teaspoon) together in a large bowl.
Scoop the tuna salad into the center of each onion ring and press down to flatten out the tops.
Evenly distribute half the cheese over the onion rings.
Bake in preheated oven for 10 minutes.
Remove baked melts from the oven and allow them to cool for about 5 minutes. Then flip them over, and sprinkle the remaining cheese on top. Return to the oven for another 10-12 minutes. You'll know they're done once the tops are golden brown.
Allow them to cool slightly on the baking sheet (this will give the cheese time to harden). Enjoy!