Purée garlic with oil and 3/4 teaspoon salt in a blender until smooth. Toss sweet potatoes with garlic oil in a large bowl, then spread in 1 layer in a 15-by 10-inch shallow baking pan.
Bake until golden in patches and cooked through, 20 to 30 minutes.
To fry sage leaves:.
Heat oil in a small heavy skillet over medium-high heat until it shimmers, then fry sage leaves in 2 batches, stirring, until crisp, 30 seconds to 1 minute per batch. Transfer with a slotted spoon to paper towels to drain.
Serve sweet potatoes with sage leaves scattered on top.
•Sweet potatoes can be cut and tossed with garlic oil 4 hours ahead and chilled in a sealable bag.
•Sage leaves can be fried 4 hours ahead and kept at room temperature.