Parboil the potatoes, drain and set aside.
Peel the sweet potatoes, then cut them into chunks. Smash the garlic cloves.
Arrange both potatoes in a baking dish, tuck the garlic cloves between them, then crumble some butter/margarine over them. Season with salt.
Roast for 15 minutes.
Meanwhile, cut the bacon into cubes. Chop the rosemary leaves.
Take the tray out of the oven, toss the potatoes, then scatter the rosemary and black pepper over them, then toss well.
Scatter the bacon on top. Drizzle with some olive oil, if needed.
Roast for another 15 minutes or so, until the bacon and the potatoes are crispy.
Serve warm.