Make Celeriac Remoulade: whisk mayonnaise, lemon juice, lemon zest, mustard and salt together in a large bowl. Peel celeriac, cut into thin matchsticks (a Japanese mandoline is handy for this), adding them to the mayonnaise mixture as soon as possible to prevent browning. Toss chives through, cover and refrigerate until needed, preferably overnight. Use a meat mallet to hammer veal as thinly as possible without breaking it, ideally about 5mm. Put flour on a large plate, egg in a wide bowl and breadcrumbs in another wide bowl and arrange them side by side in that order, with another large plate at the end. Dredge both sides of a piece of veal in the flour, shaking off excess, draw it through the egg to coat both sides, then place it in the breadcrumbs and use one hand to press them on to coat both sides well. Place on the clean plate and repeat with remaining veal pieces. Heat butter and oil in a frying pan over medium-high heat until foaming.
Add veal, sprinkle generously with salt, and cook for 2-3 minutes each side until golden.
Serve immediately on warm plates with lemon cheeks and Celeriac Remoulade on the side.