Position an oven rack so the tops of the assembled tomato rounds will be about 4-5 inches (10-13 cm) from the broiling element and preheat the broiler. In a medium-to-large mixing bowl, flake the drained tuna with a fork. Add the minced red onion, minced celery, chopped parsley, mayonnaise, Dijon mustard, lemon juice, and smoked paprika. Stir gently until all ingredients are well combined. Season with kosher salt and freshly ground black pepper according to your preference.

Carefully slice each tomato horizontally into three even slices. The top and bottom pieces will serve as the base for the tuna rounds (reserve the center slices for another use, such as a fresh salad or a sandwich). Arrange the tomato tops and bottoms, cut-side up, on a sturdy baking sheet suitable for broiling. Generously distribute the prepared tuna salad mixture evenly onto the cut surface of each tomato bottom half. Sprinkle the shredded sharp cheddar cheese uniformly over the tuna salad. If desired, you can lightly season the plain tomato top halves with a touch of salt and pepper.

Place the baking sheet under the preheated broiler. Broil for 3 to 6 minutes, watching very carefully, until the cheddar cheese is fully melted, bubbly, and has developed a rich golden-brown hue on top. Ovens and broilers vary, so keep a close eye to prevent burning. Remove from the oven promptly once the desired color is achieved.

Serve the Sunset Tuna & Tomato Rounds warm. Typically, each serving consists of two tuna-topped tomato pieces (e.g., one bottom and one top, or two bottoms depending on preference for how the "tops" are used).