1. To make the béchamel, melt the 4 tablespoons butter in a small saucepan over medium heat. Whisk in the flour and let cook for 1 minute. Add 2 cups milk, ¼ cup at a time, whisking constantly as the sauce thickens. Thin with more milk if necessary. Season generously with salt and pepper and with nutmeg to taste. Turn the heat to low and cook, whisking, for 10 to 15 minutes. Keep the sauce warm in a double boiler.

2. Meanwhile, cut the stems from the chard. Trim them and cut into batons about ½ inch thick by 3 inches. Rinse well and set aside. Stack the chard leaves about 6 at a time, roll them up like a cigar, and cut into 1-inch-wide strips. Wash twice in cold water and drain.

3. Bring 8 cups well-salted water to a boil in a saucepan. Add the chard stem batons and simmer until tender, about 5 minutes. Drain and let cool. 

4. Heat the olive oil in a wide skillet over medium-high heat. Add the garlic and red pepper flakes and let sizzle without browning, then add the chopped chard leaves. Season with salt and pepper and stir-fry until just wilted, about 2 minutes. Drain in a colander. When the chard is cool, squeeze to remove excess liquid.

5. Heat the oven to 400°F. Use 1 tablespoon of the butter to grease a 2-quart gratin dish or shallow baking dish. Add the chard leaves in an even layer. Arrange the cooked stems over the top. Spoon the béchamel over the entire dish. Sprinkle with the grated cheese and dot with the remaining 2 tablespoons butter.

6. Bake until golden and bubbling, about 25 minutes.