Toast the pecans: Heat a dry skillet over medium heat. Add pecan halves and cook, stirring frequently, until fragrant and lightly browned, about 3 minutes. Transfer to a cutting board to cool, then chop roughly.

Prepare the vegetables: Dice the celery and apple into small cubes (about 1/4 inch). If not using immediately, toss apple with a little lemon juice to prevent browning.

Make the dressing: In a large bowl, whisk together mayonnaise, Greek yogurt, Dijon mustard, lemon juice, salt, and pepper until smooth.

Combine: Add the chicken, celery, apple, and chopped pecans to the bowl with the dressing. Stir gently with a spatula until everything is evenly coated.

Taste and adjust: Add more salt, pepper, or lemon juice if needed. For best flavor, let the salad rest for 10 minutes at room temperature before serving.

Assemble sandwiches: Toast the bread if desired. Divide the chicken salad evenly among four sandwiches (about 3/4 cup each). Serve immediately.