Root Vegetable Gratin

Name: Root Vegetable Gratin
Ingredients: Good olive oil; 1 1/2 cups sliced yellow onion (1 large); 2 cups (1/4-inch-sliced) fennel, top and core removed; 1 tablespoon minced garlic (3 cloves); 1 pound sweet potatoes, peeled and sliced 1/4 inch thick; 1 pound celery root, peeled and sliced 1/4 inch thick; 1 pound Yukon Gold potatoes, peeled and sliced 1/4 inch thick; 2 1/4 cups heavy cream; 1/2 cup chicken stock, preferably homemade; 2 cups grated Gruyère cheese (6 ounces with rind); 2 teaspoons minced fresh thyme leaves; Kosher salt and freshly ground black pepper; 2 1/2 cups coarse fresh bread crumbs, crusts removed