1 Preheat the oven, prepare the baking dish: Preheat the oven to 375℉.
2 Slice the vegetables: Using a mandoline or a very sharp chef’s knife, cut the parsnips and carrots diagonally into slices about 1/8-inch thick. Halve the fennel, then cut into slices about 1/8-inch thick.
3 Sauté the aromatics and fennel: Melt the butter in a sauté pan over medium-high heat. When the butter is foamy, add the shallots, and cook until soft and translucent, about 3 minutes. Add the fennel, and cook until just tender, 4 minutes. Season with 1/2 teaspoon salt and 1/4 teaspoon pepper. Stir in the garlic and thyme, and cook until aromatic, another 30 seconds.
4 Layer the vegetables: Grease a large (2 quart) gratin dish or a 9-x-13-inch glass baking dish with butter. Layer half of the parsnips in the gratin dish, sprinkle with a pinch of salt and a grind