In a shallow dish, toss together the breadcrumbs, zest, 3/4 tsp. salt and 1/4 tsp. pepper. On a work surface, pound the thighs 1/4 inch thick. Dip each thigh into the egg whites, then coat with breadcrumb mixture and transfer to a large plate.
In a large bowl, whisk the mayonnaise and vinegar. Add the cabbage and scallions; toss well. Using a box grater, grate the apple and toss with the slaw. Season with salt and pepper. Refrigerate until ready to serve.
Heat a large, heavy skillet over medium. Add 1/4 cup EVOO. Working in batches, pan-fry the thighs until golden brown, 3 to 4 minutes per side. Transfer to a paper towel-lined plate and tent with foil to keep warm. Repeat with the remaining thighs, adding 1 or 2 tbsp. more EVOO between batches as needed.
Arrange 2 thighs on each plate and top with some of the slaw. Serve with lemon wedges and the remaining slaw.