Preheat the oven to 225 degrees F (99 degrees C). Line a large baking sheet with aluminum foil and place in the oven to preheat.
Place celeriac in a bowl; drizzle olive oil over and season with salt and black pepper. Toss fries to coat evenly.
Remove baking sheet from the hot oven. Transfer fries to the baking sheet in a single layer with some space between them.
Bake in the preheated oven for 20 minutes.
While fries bake, combine tomato paste, tomato sauce, apple cider vinegar, harissa, brown rice syrup, coriander, garlic powder, onion powder, and sea salt in a small bowl. Stir to combine until smooth. Add water, 1/4 teaspoon at a time until desired consistency. Set ketchup aside.
Remove fries from oven and increase oven temperature to 450 degrees F (230 degrees C). Flip fries and return baking sheet to the hot oven. Bake until crispy, about 15 minutes more.
