Preheat your oven to 350°F (180°C). Spread the walnuts on a baking sheet and toast for about 5 minutes until fragrant and golden brown. Allow to cool before chopping.

In a large saucepan over medium heat, add enough water to submerge the chicken breasts. Bring to a simmer and poach for 15-20 minutes until cooked through. Remove, cool, and cube.

Core and dice the red and green apples, chop the celery into thin slices, and place all these ingredients in a large mixing bowl.

Add the cubed chicken to the bowl with apples, celery, and raisins or grapes if using. Gently fold together to combine.

In a small bowl, whisk together the mayonnaise, lemon juice, and Dijon mustard until smooth. Adjust salt to taste.

Drizzle the dressing over the salad ingredients and fold gently to combine without mashing the apples and chicken.

Let the salad sit for about 10 minutes at room temperature to allow flavors to meld.

Arrange the lettuce on a serving platter and spoon the salad mixture on top. Serve and enjoy!