For the roasted sweet potatoes: Preheat the oven to 350 degrees F.
Put the sweet potato cubes on a baking sheet, drizzle with the oil and sprinkle with salt and pepper. Roast until tender, about 45 minutes.
For the mashed potatoes: Put the potatoes in a medium pot, cover with cold water and add 2 tablespoons salt. Bring just to a simmer and simmer the potatoes, uncovered, until tender, about 45 minutes. Drain.
Peel the potatoes and return them to the pot (if desired, pass them through a ricer or food mill after peeling) and add the sweet potatoes. Mash the potatoes with the milk and butter using a fork or potato masher. Stir in the Parmesan, sun-dried tomatoes and 2 teaspoons salt; season with pepper.