Season chicken breast with sea salt and black pepper.
Heat olive oil in a pan over medium heat and cook chicken until the internal temperature reaches 165 degrees Fahrenheit.
In a small non-stick skillet, combine walnuts and maple syrup, stirring constantly until the syrup thickens and coats the nuts.
Remove the candied walnuts from heat and spread them on parchment paper to cool and harden.
Dice the cooked chicken, apple, and celery into uniform pieces and halve the grapes.
In a large mixing bowl, stir together the Greek yogurt and lemon juice until smooth.
Add the chicken, apple, celery, and grapes to the bowl, tossing until everything is thoroughly coated in dressing.
Arrange the mixed greens on a plate and top with the chicken salad mixture and the crunchy candied walnuts.