Rinse chicken breasts, pat dry, and season both sides with 1/2 tsp. salt and a pinch of pepper. Heat 2 tsp. olive oil in a medium non-stick pan over medium-high heat. Place chicken in hot pan and cook until golden brown, 2-3 minutes per side. Transfer to prepared baking sheet. Wipe pan clean and reserve. Chicken will finish cooking in later step.

Place baking sheet in oven and roast until chicken reaches a minimum internal temperature of 165 degrees, 5-7 minutes. Transfer chicken to medium mixing bowl and allow to cool at least 5 minutes.

In pan used to sear chicken, combine sugar, walnuts, a pinch of salt, and 2 tsp. water over medium-high heat. Bring to a simmer and cook, stirring constantly, until most liquid has evaporated and walnuts are sticky, 5-7 minutes. Carefully (hot nut alert!) remove to a plate. Allow to cool before breaking into smaller pieces.

Zest lemon, halve, and juice. Trim ends off celery and cut into 1/4" dice. Core apple and cut into 1/4" dice. Separate leaves of lettuce for cups.

Once chicken has cooled, use two forks or your hands to pull into bite-sized shreds. Add 1 Tbsp. lemon juice, celery, apple (reserve a pinch for garnish), dried cranberries (reserve a pinch for garnish), mayonnaise, 3/4 the Greek yogurt, 1/4 tsp. salt, and a pinch of pepper to bowl with chicken. Mix ingredients together until fully combined. Taste, and add more yogurt if desired.

Place a lettuce cup on work surface (or layer two leaves for extra crunch and structural support). Fill with a few spoonfuls of chicken salad. Repeat for each lettuce cup. Arrange three or four assembled lettuce cups on a plate. Garnish with reserved apples, reserved dried cranberries, candied walnuts, and lemon zest.