Preheat oven to 425 degrees.\ Arrange the sweet potatoes and garlic in a shallow flameproof baking dish large enough to hold them in a single layer. Add just enough water to cover the vegetables. Bring the water to a boil over medium high heat and simmer for 2 minutes. Drain in a colander.
In a baking dish, combine the oil and butter and melt the butter over medium heat. Remove from the heat, add the potatoes and garlic, rosemary, and salt and pepper to taste and toss to coat. Spread the potatoes in a single layer and roast on the lowest rack of the oven, turning them occasionally, for 25 to 30 minutes, or until golden brown. Transfer to a heated serving bowl and serve at once.
Recipe courtesy of Georgia Downard, Reasons to Roast