Preheat the oven to 200 degrees Celsius (392 degrees Fahrenheit).
Peel the celeriac and cut it into 1 cm thick slices.
Place the slices on a baking sheet and brush both sides with olive oil.
Bake the celeriac in the middle of the oven for 8 minutes.
Take them out and let them cool.
Mix flour, paprika, onion powder, and salt in a bowl.
Add water and mustard and whisk it into a smooth, slightly thick batter.
Heat a pan to medium-high, pour oil in to cover the bottom, and add some of the butter.
Dip the celeriac slices in the batter and coat them in breadcrumbs.
Fry the celeriac slices on both sides and place them on a plate with paper towels.