How to Make It
Preheat oven to 400°F. Stir together tuna, celery, onion, capers, cornichons, lemon zest, lemon juice, thyme, salt, mustard, Aleppo pepper, and coriander in a medium bowl. Fold in mayonnaise. Chill 15 minutes.
Heat a grill pan over medium-high. Grill bread slices until lightly charred, 2 to 3 minutes per side.
Spread each bread slice with a heaping 1/2 cup tuna mixture, and top with 2 cheese slices.
Place tartines on a baking sheet, and bake in preheated oven until cheese is just melted, 6 to 8 minutes.
Cut in half diagonally, and serve with pickled vegetables.