Root Vegetable Gratin

Name: Root Vegetable Gratin
Ingredients: 1 tablespoon plus 1 1/2 cups water; 1 1/2 teaspoons Dijon mustard; 2 teaspoons all-purpose flour; Salt and pepper; 2/3 cup dry white wine; 1/2 cup heavy cream; 1/2 onion, chopped fine; 1 1/4 teaspoons minced fresh thyme; 1 garlic clove, minced; 2 pounds Yukon Gold potatoes, peeled and sliced lengthwise 1/8 inch thick; 1 large celery root (1 pound), peeled, quartered, and sliced 1/8 inch thick; 1 pound rutabaga, peeled, quartered, and sliced 1/8 inch thick; 3/4 cup panko bread crumbs; 1 1/2 ounces Parmesan cheese, grated (3/4 cup); 4 tablespoons unsalted butter, melted and cooled