Curried Chicken Sandwich

INGREDIENTS

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