Preheat the oven to 220 C / Gas mark 7. Lightly grease a baking tray.
Cook the potatoes in a pot of salted boiling water until tender; drain. Mash with the egg yolk and season with salt and pepper.
Meanwhile, melt the butter in a large pan over medium heat. Cook the onion, hazelnuts and cumin seeds until the onion is golden brown, 7 to 10 minutes. Stir in the turmeric, pepper, tomato puree and lemon juice; cook an additional 5 minutes. Stir this mixture into the mashed potatoes.
Take a scoop of the mash and form into croquettes; dip in the egg white and roll in coconut. Place on the prepared baking tray.
Cook in the preheated oven until lightly golden, 20 to 25 minutes. Garnish with chopped parsley.