Cut the fingerling potatoes in half lengthwise. Larger potatoes can be cut into quarters lengthwise. Spread potatoes on a sheet pan.
Drizzle the oil over the potatoes and toss to coat. Cut the lemons in half and squeeze the juice of one lemon over the potatoes. Arrange all of the lemon halves and sprigs of rosemary around the potatoes. Season with salt and pepper.
Bake for 20 minutes. Toss the potatoes and bake for another 5 minutes on the top rack until potatoes are lightly brown.