Step 1First make the rémoulade. Cut the celeriac into thin slices, then cut the slices into matchstick-sized pieces. Alternatively, you can coarsely grate the celeriac. Put into a bowl and mix in the remaining ingredients and some seasoning. Cover and chill until needed.
Step 2For the chicken schnitzel, working 1 chicken breast at a time, slice in 1/2 through its depth, but leaving 1 side attached. Open out and place between 2 sheets of baking parchment. Bash with a rolling pin to an even thickness. Repeat with remaining breasts.
Step 3Next put the flour, eggs and breadcrumbs into 3 separate shallow bowls. Mix some seasoning into the flour. Dip the flattened chicken pieces first into the flour to coat and gently tap off excess. Next fully coat in egg, and finally the breadcrumbs, pressing them on to cover, if needed.
Step 4To fry, pour enough oil into a large frying pan so it's 5mm deep. Heat over medium heat for a few min. Add the breaded chicken pieces (in batches if needed) and fry for 3min per side, or until golden and cooked through. Using tongs, transfer to a board lined with kitchen paper. Fry remaining chicken, monitoring the oil temperature.
Step 5Meanwhile, toast the bagels and place the bottoms on to 4 plates. Top with rocket and some tomato slices. Lay on the chicken schnitzels, spoon on some rémoulade and top with the bagel lids. Serve with any remaining rémoulade.