Place oven rack in upper 1/3 of oven. Preheat oven to 500 degrees F.
In a large bowl, add sliced potatoes, melted butter, oil, chopped rosemary, salt and pepper. Toss to coat all the potatoes well.
In a 9x13 metal baking pan, arrange the potatoes in a single layer. Roast, turning once, until browned, about 30 minutes.*DO NOT USE A GLASS DISH! It may shatter.
Remove dish from oven. Carefully add broth, lemon juice and zest, garlic and 2-3 rosemary sprigs to the pan evenly. Continue roasting until most of the liquid is absorbed and the potatoes are very tender, about 10 to 12 minutes.
Discard sprigs. Top with additional rosemary as a garnish, if desired.