Curried Chicken Sandwich

Name: Curried Chicken Sandwich
Ingredients: 2 skinless, boneless chicken breasts (about 1 pound), halved horizontally; 1/4 recipe Curry Brine; 4 stalks celery, thinly sliced; 1/2 small fennel bulb, thinly sliced; 1 tablespoon plain whole-milk yogurt; 1 teaspoon fresh lemon juice; 3/4 teaspoon celery seeds; 1 tablespoon mayonnaise plus more for serving; Kosher salt, freshly ground pepper; freshly ground pepper; 4 large or 8 small slices country-style bread; 1/4 cup olive oil; 1/2 small red onion, thinly sliced; 1/2 cup picked dill with tender stems