In a bowl, gently but thoroughly combine all egg salad ingredients. Taste and adjust as needed. Refrigerate until ready to use.
In a separate bowl, add lemon juice to sliced celery along with sugar and salt and toss until well-combined. Refrigerate until ready to use.
Bring a large pot of water to a boil. Place sliced celery root on a wire rack over the pot of water and steam until softened, about 20 minutes (you can cover with aluminum foil to speed up this process). Thoroughly dry the pot, add oil for frying, and heat until 350°F (it should be shimmering). Set up your dredging station. To the breadcrumbs, whisk in white pepper and onion powder. Coat each celery root coin with all purpose flour, then egg, and finally seasoned breadcrumbs. Fry in batches until golden brown, transfer to a drying rack, and sprinkle with salt.
Transfer celery root cutlets to a serving plate. Add a big scoop of egg salad. Use a slotted spoon to drain pickled celery before adding to the plate, too. Enjoy some of everything with each bite.