Bring a large pot of salted water to a boil over high heat. Slip in the beans and cook until bright green and crisp-tender. Remove the beans to an ice bath to shock, then drain and pat dry with paper towels.
Gently heat the olive oil and rosemary in a large, deep skillet over low heat until the rosemary is softened and very fragrant, 4 to 5 minutes. Remove the rosemary sprig, raise the heat to medium-high, and add the string beans. Toss to coat with the rosemary-lemon scented oil until the beans are heated through. Season with salt and pepper, to taste. Serve warm or at room temperature.
Lemon-Infused Olive Oil:
Put the lemon zest in a small pot and pour in 2 cups extra-virgin olive oil. Place over low heat and simmer for 15 minutes. Cool completely and transfer to a jar with a tight-fitting lid. Keep tightly sealed at room temperature for up to 2 weeks. Dress grilled chicken or any simply vegetable with the oil. When tomatoes are in season, a simple dish of sliced tomato dressed with this oil and a sprinkling of oregano is a meal unto itself.