Place the chicken breast in a small pot of simmering water and cook for 12-15 minutes until fully cooked through, then let cool and dice into bite-sized pieces.
Slice the red grapes in half, finely dice the celery, and thinly slice the green onion.
In a dry skillet over medium heat, lightly toast the pecan halves for 2-3 minutes until fragrant, then roughly chop.
In a medium mixing bowl, whisk together the Greek yogurt, avocado oil mayonnaise, Dijon mustard, lemon juice, sea salt, and black pepper until smooth.
Add the cooled chicken, grapes, celery, green onion, and pecans to the bowl with the dressing.
Fold all ingredients together until evenly coated and serve immediately or chill for 30 minutes to let flavors meld.