Step 1: Prepare the Swiss Chard. Wash the swiss chard thoroughly. Separate the leaves from the stems. Chop the stems into 1/2-inch pieces and coarsely chop the leaves.
Step 2: Sauté the Aromatics and Chard Stems. In a large, oven-safe skillet or Dutch oven, heat the olive oil over medium heat. Add the chopped onion and cook until softened, about 5-7 minutes. Add the chopped swiss chard stems and minced garlic, cooking for another 3-4 minutes until the stems begin to soften. Pro Tip: Cooking the stems first ensures they are tender when the dish is finished.
Step 3: Wilt the Chard Leaves. Add the chopped swiss chard leaves to the skillet in batches if necessary, stirring until they wilt down. This usually takes about 5 minutes. Stir in the rinsed cannellini beans and dried thyme. Season with salt and pepper. Remove the mixture from the skillet and set aside.
Step 4: Prepare the Béchamel Sauce. In the same skillet (no need to clean), melt the butter over medium heat. Whisk in the flour and cook for 1-2 minutes, stirring constantly, to create a roux. Gradually whisk in the warmed milk, a little at a time, until the sauce is smooth and thickened. Continue to cook, stirring, until the sauce coats the back of a spoon, about 5-7 minutes. Pro Tip: Warming the milk helps prevent lumps when adding it to the roux.
Step 5: Finish the Sauce. Remove the béchamel from the heat. Stir in the freshly grated nutmeg, 1 cup of sharp cheddar cheese, and 1/2 cup of Parmesan cheese until melted and smooth. Taste and adjust seasoning with salt and pepper.
Step 6: Combine and Bake. Return the swiss chard and white bean mixture to the skillet with the béchamel sauce. Stir gently to combine everything thoroughly. Smooth the top and sprinkle with an additional layer of Parmesan cheese. Bake in a preheated oven at 375°F (190°C) for 25-35 minutes, or until bubbly and golden brown on top. Pro Tip: For an extra golden crust, you can finish it under the broiler for 1-2 minutes, watching carefully to prevent burning.
Step 7: Serve. Let the gratin rest for 5-10 minutes before serving to allow the sauce to set slightly. Serve warm and enjoy!