Preparation Position rack in center of oven and preheat to 350°F. Butter 13x9x2-inch glass baking dish. Place garlic cloves and rosemary sprigs in bottom of prepared baking dish. Combine cream, coarse salt, and white pepper in large bowl. Cut off any stalks and fronds from fennel bulbs; discard stalks. Finely chop fennel fronds; cover and chill. Using V-slicer or mandoline, thinly slice fennel bulbs. Layer half of fennel slices evenly atop garlic and rosemary in baking dish. Peel potatoes and place immediately in large bowl of cold water to prevent discoloration. Working with 1 potato at a time, use a V-slicer or mandoline to thinly slice into rounds. Immediately add potatoes to bowl with cream mixture, turning to coat potatoes. Arrange half of potato slices evenly over fennel slices in baking dish, reserving cream mixture in bowl. Repeat layering with remaining fennel slices, then remaining potato slices. Pour cream mixture in bowl over fennel-potato mixture in baking dish. Dot with butter cubes. Cover baking dish with foil, doming foil slightly (do not allow foil to touch potatoes). Bake scalloped potatoes until almost tender when pierced with knife, about 1 hour 15 minutes. Increase oven temperature to 425°F. Remove foil and bake uncovered until potatoes are tender and top is deep golden brown, about 20 minutes longer. Remove from oven and let rest 15 minutes. Sprinkle with reserved fennel fronds and serve.
Preparation Position rack in center of oven and preheat to 350°F. Butter 13x9x2-inch glass baking dish. Place garlic cloves and rosemary sprigs in bottom of prepared baking dish. Combine cream, coarse salt, and white pepper in large bowl. Cut off any stalks and fronds from fennel bulbs; discard stalks. Finely chop fennel fronds; cover and chill. Using V-slicer or mandoline, thinly slice fennel bulbs. Layer half of fennel slices evenly atop garlic and rosemary in baking dish. Peel potatoes and place immediately in large bowl of cold water to prevent discoloration. Working with 1 potato at a time, use a V-slicer or mandoline to thinly slice into rounds. Immediately add potatoes to bowl with cream mixture, turning to coat potatoes. Arrange half of potato slices evenly over fennel slices in baking dish, reserving cream mixture in bowl. Repeat layering with remaining fennel slices, then remaining potato slices. Pour cream mixture in bowl over fennel-potato mixture in baking dish. Dot with butter cubes. Cover baking dish with foil, doming foil slightly (do not allow foil to touch potatoes). Bake scalloped potatoes until almost tender when pierced with knife, about 1 hour 15 minutes. Increase oven temperature to 425°F. Remove foil and bake uncovered until potatoes are tender and top is deep golden brown, about 20 minutes longer. Remove from oven and let rest 15 minutes. Sprinkle with reserved fennel fronds and serve.