Add crunch and flavor: Stir in the chopped celery leaves or extra celery, dill pickle or relish, and red onion or green onion. Sprinkle in the garlic powder, onion powder, salt, and black pepper. Mix until everything is evenly combined, then taste and adjust seasoning if needed.
Brighten with lemon: Add 1 tablespoon of lemon juice and stir again. Taste the tuna salad—if you like it tangier, add up to 1 more tablespoon of lemon juice, a little at a time.
Fill the celery boats: Using a spoon, scoop generous mounds of tuna salad into the hollow of each celery piece, pressing lightly so it stays put and piling it just a bit higher than the edges for that classic, creamy look. Keep going until all the celery pieces are filled. If you have extra tuna salad, either spoon it a bit higher on each piece or save it in a small container for later.
Finish and chill: If you like, lightly dust the tops with paprika for color. Slide the sheet pans into the refrigerator and chill for at least 30 minutes so the celery stays crisp and the tuna salad is nice and cold—just like Aunt Denise’s after-school stash.
Serve or store: Serve directly from the chilled sheet pans, or transfer the celery boats to a platter. For easy snacking through the week, cover the sheet pans tightly with plastic wrap or foil and keep them in the fridge for up to 3 days.
