Light a grill or preheat the broiler. In a large skillet, heat 3 tablespoons of the olive oil over high heat. Add the onions, garlic, rosemary, vinegar and honey and cook until the onions soften, about 4 minutes. Season with salt and pepper. Cover and cook over low heat until the onions are very tender, about 10 minutes. Add the plums to the skillet, cover and cook until the plums are tender, about 8 minutes. Discard the rosemary sprigs.