Drain the tuna thoroughly and flake it into a medium bowl. Add the mayo, celery salt, minced shallots, pickle, lemon juice, Dijon mustard, minced garlic, salt, and pepper. Fold everything together gently until just combined—don't overmix, as you want to maintain some texture in the tuna. I find that freshly squeezed lemon juice makes a noticeable difference in brightness compared to bottled, so it's worth the extra minute. Set the tuna salad aside while you prepare the rolls.

Preheat your oven to 350°F and lightly grease a 9x13-inch baking dish (or similar size that fits your rolls snugly). Carefully separate the Hawaiian rolls by slicing them horizontally—keep the bottom halves in place in the dish and set the tops aside. Divide the tuna salad from Step 1 evenly among the bottom halves, spreading it gently with a spatula. Top each tuna-filled roll with one slice of Havarti cheese, then cover with the corresponding roll top.

In a small bowl, combine the melted butter with the celery seed. Brush this mixture evenly over the tops of the assembled rolls. Cover the dish tightly with foil and bake for 10–15 minutes until the cheese begins to melt and the interior is heated through. Then remove the foil and bake uncovered for an additional 5 minutes to lightly toast the tops and allow any remaining moisture to evaporate. I always let the rolls cool for just 2–3 minutes after coming out of the oven so they hold together better when you pick them up.

Remove the sliders from the oven and sprinkle the fresh parsley over the top for a bright, fresh finish. Serve immediately while the cheese is still melted and the rolls are warm.