2 cups crisp apples (such as Gala or Fuji), diced
1 tbsp lemon juice (to prevent browning)
1 cup celery, thinly sliced
1 cup red grapes, halved (or green grapes)
½ cup walnuts, roughly chopped
½ cup mayonnaise or plain Greek yogurt (for a lighter option)
Salt and black pepper to taste
Optional: lettuce leaves for serving