Put the sweet potatoes and baking potatoes in separate pans of cold salted water, cover them and bring to a boil. Boil until they are just tender, 15-20 minutes. Drain both pans and keep the sweet potatoes warm.
Return the baking potatoes to the pan and crush them with a potato masher. Add the onion, chili pepper, lemon juice, olive oil and red pepper flakes. S