Peel and clean the celeriac root. Wash the oranges, grate the zest, and squeeze the juice from one half. Slice the other half into very thin slices. Using a sharp knife or a mandoline, cut the celeriac and orange into paper-thin slices.

Arrange the celeriac slices on a plate, then layer the orange slices on top.

Make the vinaigrette by mixing the orange juice with the mustard (taste before adding more), olive oil, salt, and chives.