Preheat the oven to 200°C.
Peel and dice the onion and the celeriac*.
Add the vegetables to a large baking dish (one that is wide and not too deep).
Add the olive oil, salt, nutmeg and garlic powder. Mix thoroughly with your hands. Bake for 30 minutes.
In the meantime, grind the oats and cashews in a small blender or food processor.
Add the oats, cashews, apple sauce, maple syrup, coconut oil and salt to a bowl. Mix with you hands until you reach a crumbly consistency.
Dry-roast (without oil) the sunflower seed in a pan or skillet over high heat until golden. Watch carefully. Add it to the crumble mixture.
After 30 minutes, sprinkle the crumble mixture over the celeriac. Bake for another 10 to 15 minutes.