Step 1

Pack cucumber, habanero, and dill into a heatproof glass jar; set aside. Bring vinegar, sugar, kosher salt, red pepper flakes, celery salt, mustard seeds, coriander, turmeric, and ½ cup water to a boil in a medium saucepan, stirring to dissolve sugar and salt, then pour into jar. Cover and let cool; chill.

Step 2

DO AHEAD: Pickles can be made 2 weeks ahead. Keep chilled.

Step 3

Preheat oven to 425°. Mix tuna, celery, cucumber, onion, oil, lemon juice, mustard, chives, dill, celery salt, and ¼ cup mayonnaise in a medium bowl; season with salt, pepper, and hot sauce.

Step 4

Place bread on a baking sheet and spread remaining mayonnaise over 4 slices; top with tuna salad and cheese. Bake until cheese is melted, about 4 minutes. Top melts with lettuce and pickles and close.