Root Vegetable Gratin

Name: Root Vegetable Gratin
Ingredients: Nonstick cooking spray, for the baking dish; 1 1/4 cups heavy whipping cream; 1 1/4 cups chicken stock; 2 tablespoons salted butter; 10 sprigs fresh thyme; 4 cloves garlic; 1 Parmesan rind; 2 tablespoons all-purpose flour; 3 large Yukon gold potatoes (about 1 pound), peels on, thinly sliced; Kosher salt and freshly ground black pepper; 1/2 pound manchego cheese, grated; 3 thick carrots (about 1 pound), thinly sliced; 3 thick parsnips (about 1 pound), thinly sliced