For the grilled grapes: Drizzle the bunched grapes with the olive oil, turning to coat. Place in a grill basket on the grill and cook, gently turning to cook evenly, until the grapes are slightly soft and beginning to char and burst, 3 to 4 minutes. When cool enough to handle, remove the grapes from the stems and set aside.
For the dressing: Meanwhile, in a large bowl, whisk together the honey, Dijon mustard and lemon juice. Continue to whisk while slowly streaming in the olive oil. Season with salt and pepper.
For the salad: Add the celery and chicken to the bowl with the dressing. Core, halve, then slice the apple into 1/4-inch slices, and add to the bowl. Add the walnuts and cooled grapes to the bowl and toss to coat everything with the dressing.
To serve, line a salad plate with the lettuce leaves. Spoon a quarter of the chicken salad over the top of each leaf. Serve immediately.