Lightly coat grill or grill pan with cooking spray. Preheat to medium-high heat. Peel onion and slice into ½" rounds. Zest lemon, halve, and juice. Stem and mince mint. Trim cucumber, quarter lengthwise, then cut into ¼" pieces. Season both sides of chicken breasts with a pinch of salt and seasoning blend. Drizzle chicken with ¼ tsp. olive oil.
Make the Couscous
Place couscous in a mixing bowl with a pinch of salt. Pour boiling water on top. Stir 10 seconds, cover, and set aside 10 minutes. Uncover and fluff. While couscous sits, grill onions.
Grill the Onions
Place onions on grill. Flip occasionally until charred, 5-6 minutes. Transfer to a plate. Set aside.
Make the Vinaigrette
Combine honey, 2 tsp. lemon juice, mint (to taste), 2 tsp. olive oil, and a pinch of salt and pepper in a mixing bowl. Set aside to let flavors marry.
Grill the Chicken
Place chicken on grill. Cook undisturbed 3-4 minutes. Flip and cook until chicken is charred and reaches a minimum internal temperature of 165 degrees, 3-4 minutes. Remove from grill and rest 3 minutes. Don't worry if chicken becomes very charred; it's supposed to!
Finish the Dish
Finely chop onions. Add cucumbers and onions to couscous. Toss to combine. Serve chicken on a plate alongside couscous. Garnish couscous with lemon zest (to taste). Drizzle vinaigrette over chicken.