Root Vegetable Gratin

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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