Peel the potato and cut it into very small pieces. Heat the butter in a frying pan, and fry chopped potato for 5 minutes. Peel and finely chop the onion. Add onion to the frying pan and continue to fry over medium heat for 10 minutes, or until potato and onion are tender and slightly brown.

In a meantime, bring a pot of water with a pinch of salt to a boil. Add frozen peas and cook over medium heat for 6-8 minutes, drain and set aside.

Transfer cooked peas, potato, and onion to a blender. Add fresh parsley, season with salt and black pepper, blend until silky smooth. Transfer your soup to the pot and warm up to the desired temperature. Serve with a spoon of sour cream and nigella seeds.