Preheat the oven 450F. Place a wire rack on a foil-lined baking sheet, and set it aside.
Put the flour in a shallow dish. Mix the panko and poultry seasoning together in another shallow dish. In a medium bowl, whip the egg whites with a whisk until they are extremely foamy, but not quite holding peaks.
Season the chicken with salt and pepper to taste. Working in batches, dredge the chicken tenders in the flour, shaking off any excess. Add the chicken to the egg whites and toss to coat them completely. Add the chicken, a few pieces at a time, to the bowl of panko and coat completely.
Spread the chicken on a wire rack. Season well with the salt and pepper, and spray lightly with cooking spray.
Bake until the breading is golden and crispy, and the chicken is cooked through, about 10 minutes.
Meanwhile, in a small bowl, mix the salad dressing mix with the sour cream and buttermilk.
Serve the chicken tenders with the ranch dressing.