Celery Root and Hash Brown Cake

Ingredients

• A scant 1 pound of potatoes
• 1 celery root (about 12 oz or more)
• 4 tablespoons sunflower seed oil, butter or a mixture
• 1 medium onion or 2 shallots, finely diced
• Salt and freshly ground pepper
• A few pinches of celery seed
• 1/2 cup grated Gruyere cheese
• Finely chopped parsley or celery leaves, to finish