Scrub the sweet potatoes clean and prick them all over with a fork. Microwave according to your manufacturers directions until the potatoes are tender. Cool slightly and peel.
Heat the olive oil in the pot and swirl it around. Add the leeks and sauté until soft, about 2 minutes. Add the cumin, merkén, vegetable stock, and salt and bring to a boil. Reduce heat to a simmer and add the sweet potatoes, mashing in gently. Puree the soup with the immersion blender and allow to simmer for about 3 to 4 minutes. Ladle into bowls and garnish with cheese, parsley, and merkén to taste.