Preheat the oven to 350 degrees F.
For the dressing: Whisk together the yogurt, lemon juice, oil, vinegar, thyme, 1/2 teaspoon salt and 1/4 teaspoon pepper in a small bowl until combined; set aside.
For the salad: Spread the walnuts on a baking sheet, and toast until lightly browned and fragrant, 5 to 7 minutes. Let cool for 5 minutes, then break up into 1/4-inch pieces.
To assemble: Toss the walnuts, chicken, apples, grapes and scallion whites together with the dressing in a large bowl until everything is well coated; season with salt and pepper. Scoop a scant 1/2 cup of the salad into each romaine leaf, and arrange 2 on each serving plate. Garnish with the scallion greens, and serve.
Never leave a child unattended in the kitchen. Limit the child to tasks that are safe and age-appropriate.
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