I love grilled potatoes. Even in the foil packet, they take on a beguiling smokiness from the grill, which stands out all the more against the sweetness of sweet potatoes. You could mince the rosemary and toss it with the potato slices, but you don't really need to; you'll be surprised by how much those two sprigs on the outskirts infuse the whole packet.
1 pound orange-fleshed sweet potatoes (also called yams), peeled and cut into 1/4-inch slices
2 tablespoons extra virgin olive oil
sea salt and freshly ground black pepper
2 sprigs rosemary
Preheat grill to medium-high.
Toss sweet potatoes with olive oil, salt and pepper.
Lay out a 20-inch piece of heavy duty foil. Spread the sweet potatoes onto one half of the foil and place rosemary sprigs on either side. Fold the other half over the top and crimp the edges up and over to seal well. If needed, wrap another piece of foil around each to reinforce.
Place foil packets directly over heat, cover and cook for 25 minutes, flipping once with tongs and a wide spatula, and moving around grill every once in a while.
To serve, open packets and discard rosemary.