Jerusalem Artichoke Gratin

Name: Jerusalem Artichoke Gratin
Ingredients: 1 pound Jerusalem artichokes, peeled and sliced 1/4-inch-thick and reserved in cold water until ready to use; 3 cups milk; 8 ounces creme fraiche; 2 tablespoons freshly squeezed lemon juice; 1 cup grated Gruyere cheese; 1 tablespoon minced fresh thyme; 1 teaspoon coarse salt; 1/4 teaspoon freshly ground pepper; 1/2 pound Yukon gold potatoes, peeled and sliced 1/4-inch-thick; 5 ounces shallots, peeled and sliced thinly; 5 ounces peeled chestnuts, halved lengthwise; 4 slices white bread, lightly toasted, crusts removed, and torn into small bits (to make 1 to 1 1/4 cups)