1. Add potatoes to a large pot of lightly salted water. Bring to a boil and boil until soft. Peel potatoes and mash until smooth.
2. Heat oil in a sauté pan over medium heat. Add onion and sauté until soft. Add onion, two tablespoons oil, salt, and parsley to potatoes and mix well.
3. Add sweet potatoes to a large pot of lightly salted water. Bring to a boil and boil until just slightly softened. Drain.
4. Heat olive oil in a sauté pan and sauté sweet potatoes. Add garlic, black pepper, and salt, and sauté about one minute. Add parsley, sauté another two minutes, and remove from heat.