Preheat the oven to 400 degrees . In a large bowl, combine the chicken, onion, 1/3 cup buffalo sauce and the olive oil; season with salt and pepper. Transfer to a foil-lined rimmed baking sheet. Bake, stirring the onion and turning the chicken halfway through, until the onion is softened and browned and the chicken is cooked through, about 35 minutes.
Meanwhile, in a salad bowl, whisk together the blue cheese, sour cream and lemon juice; season with salt and pepper. Add the celery and mixed greens and toss. Top the salad with the baked chicken and onion. Serve with more buffalo sauce on the side