To make fresh bread crumbs, trim crusts from sliced bread, tear bread into pieces and process in a food processor until coarse crumbs form. Usually, one slice of bread makes about 1/2 c fresh crumbs. Although onion can be considered a “root” vegetable, it is a separate ingredient in this dish.
3 lbs assorted root vegetables, (peeled) and cut into very thin slices
3 T olive oil, divided, plus extra for greasing baking dish
1 c thinly sliced onion, or shallots
1 1/3 c milk, divided
3 T all-purpose flour
1 1/2 c finely shredded Gruyère cheese, divided
1 T chopped fresh thyme or 1 t dried
salt and pepper, to taste
1 c fresh (whole-wheat) bread crumbs
1. Preheat oven to 400ºF. Lightly oil a 9-by-13-inch baking dish. Bring a large pot of water to the boil.
2. Cook sliced vegetables in the boiling water until barely tender, about 5 minutes. Drain.
3. Heat 2 T of the oil in a saucepan over medium heat. Add onions and cook, stirring occasionally, for about 8 minutes (shallots will take 3 to 4 minutes). Add 1 cup milk and bring to a simmer. Combine the remaining 1/3 c milk and the flour in a small bowl to make a smooth paste. Stir this into the hot milk and cook, whisking constantly, until the sauce bubbles and thickens, a few minutes. Remove from heat. Stir in 3/4 cup cheese, thyme, salt, and pepper.
4. Combine the bread crumbs, the remaining 3/4 cup cheese and 1 T olive oil in a bowl.
5. Layer the vegetable slices in the prepared baking dish. Pour the cheese sauce over, then top with the bread crumb mixture. Bake until the gratin is bubbling, the top is golden, and the vegetables are tender, 30 to 40 minutes (or longer, depending on the thickness of your slices). Let stand for 10 minutes before serving.