This gratin will convert those wary-of-green-vegetable eaters to greens lovers. A simple white sauce lightly coats Swiss chard leaves, and a topping of cheese and crisp breadcrumbs seals the deal. For a different presentation, bake in individual gratin dishes or ramekins rather than one large dish. Make up to a day ahead and keep in the fridge until ready to serve.
Preheat oven to 350 degrees. Place chard leaves in a saucepan with the water and cook over medium heat until leaves are just tender, 3 to 4 minutes. Drain, reserving 1/4 cup of the cooking liquid. Set chard aside. In the same saucepan, heat olive oil and butter over medium heat. When butter has melted, whisk in the flour until blended. Whisk constantly for 1 minute. Slowly whisk in the milk and reserved cooking liquid.
Continue cooking and stirring until the sauce thickens, 3 to 5 minutes. Season with salt and pepper and stir in half of the grated cheese. Stir in the cooked chard and transfer to a buttered 9-by-9-inch baking dish. Sprinkle with remaining cheese and breadcrumbs. Bake for 20 minutes or until hot and bubbling. Serve immediately.