How to Make It
Preheat oven to 350ºF. Combine cream, flour, salt, pepper, garlic, and bay leaf in a medium saucepan over medium. Cook, stirring often, until hot and slightly thickened, about 5 minutes; do not boil. Reduce heat to low while assembling gratin.
Thinly slice squash, turnips, and rutabaga into 1/4- to 1/8-inch-thick slices using a mandoline. Layer one-third of the vegetables in a lightly greased (with cooking spray) 13- x 9-inch baking dish. Top with one-third of the Gruyère. Repeat layers twice. Pour cream mixture over vegetables.
Cover with foil; bake in preheated oven until bubbly, about 1 hour and 20 minutes. Uncover, increase oven temperature to broil, and broil until top begins to brown, about 4 minutes. Remove from oven; let stand at room temperature 10 minutes.
Meanwhile, heat oil in a small skillet over medium. Working in small batches, fry sage leaves until edges begin to curl and leaves just begin to brown. Transfer to a plate lined with paper towels to drain, and cool slightly. Arrange sage leaves over gratin before serving.