Line a baking sheet with parchment paper and arrange the celery slices.
Brush with olive oil (or drizzle generously over each slice).
Season with salt and pepper and sprinkle with zaatar.
Bake for around 25 minutes (if necessary, remove the smallest / thinnest slices before they darken too much).
Check for doneness (flesh should be tender) and keep warm.
Notes
You could try different seasonings: for example, add a dash of honey or maple syrup for a sweet touch just before serving, or a drizzle of hazelnut oil.