Peel the sweet potatoes, cut into small pieces and cook in salted water until tender. Peel the onion, cut into small pieces and sauté in a little olive oil until translucent.
Process the soft-boiled sweet potatoes and butter into a puree using a potato masher. Season with sea salt and pepper from the mill. Fold the steamed onions into the puree. If you like, stir some grated Parmesan cheese into the puree.