Curried Chicken Sandwich

Name: Curried Chicken Sandwich
Ingredients: 2 skinless, boneless chicken breasts (about 1 pound), halved horizontally; 1 /4 recipe Curry Brine (click for recipe); 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; 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