Preparation Combine chicken and Curry Brine in a large resealable plastic bag, seal, and turn to coat. Chill at least 4 hours. Prepare grill for medium-high heat. Combine celery, fennel, yogurt, lemon juice, celery seeds, and 1 tablespoon mayonnaise in a medium bowl; season with salt and pepper and toss to combine. Set slaw aside. Grill chicken until cooked through, 5-7 minutes per side. Brush bread on both sides with oil and grill until toasted, about 2 minutes per side; spread with mayonnaise. Build sandwiches with bread, chicken, slaw, onion, and dill. DO AHEAD: Chicken can be brined 1 day ahead. Keep chilled.
Preparation Combine chicken and Curry Brine in a large resealable plastic bag, seal, and turn to coat. Chill at least 4 hours. Prepare grill for medium-high heat. Combine celery, fennel, yogurt, lemon juice, celery seeds, and 1 tablespoon mayonnaise in a medium bowl; season with salt and pepper and toss to combine. Set slaw aside. Grill chicken until cooked through, 5-7 minutes per side. Brush bread on both sides with oil and grill until toasted, about 2 minutes per side; spread with mayonnaise. Build sandwiches with bread, chicken, slaw, onion, and dill. DO AHEAD: Chicken can be brined 1 day ahead. Keep chilled.