Soak the red onion in a small bowl of ice water, 10 minutes. Meanwhile, combine the kale, 1 tablespoon vinegar and a pinch of salt in a large bowl. Toss and rub with your fingers until the kale is dark green and softened, 3 to 5 minutes.
Whisk the mayonnaise, buttermilk, garlic, dill and the remaining 1 tablespoon vinegar in a small bowl until smooth; season with salt and pepper. Drain the red onion, pat dry and add to the bowl with the kale; add the turkey, apple and half of the sunflower seeds. Drizzle with about three-quarters of the dressing and toss to coat; season with salt and pepper. Top with the remaining dressing, sunflower seeds and the avocado.