Instructions:
- Thoroughly wash the Swiss Chard. Separate the thick ribs from the leaves. Roughly chop the ribs and coarsely chop the leaves. Set aside separately.
- Heat olive oil in a large skillet over medium-high heat. Add sausage slices and cook until nicely browned and slightly crisp on the edges (about 5-7 minutes). Remove sausage with a slotted spoon and set aside, leaving the rendered fat in the pan.
- Add the diced onion to the skillet and cook until softened (about 5 minutes). Add the chopped chard ribs and sauté for 3 minutes until slightly tender. Stir in the minced garlic and cook for 1 minute more until fragrant.
- Add the chopped chard leaves to the skillet. Sauté until they have completely wilted down, stirring frequently. Season lightly with salt and pepper. Remove the mixture from the heat and stir the cooked sausage back in.
- In a small saucepan, melt the butter over medium heat. Whisk in the flour and cook for 1 minute (the roux). Gradually whisk in the cold milk until smooth. Bring to a gentle simmer, whisking constantly, until the sauce thickens enough to coat the back of a spoon (about 5-7 minutes).
- Remove the sauce from heat. Stir in the nutmeg, ¼ cup Parmesan, and the Gruyère/Cheddar. Taste and adjust seasoning.
- Preheat your oven to 375°F (190°C). Transfer the sausage and chard mixture to the bottom of an 8x8 inch baking dish. Pour the creamy sauce evenly over the top.
- Sprinkle the remaining ¼ cup of Parmesan evenly over the surface. Bake for 20–25 minutes, or until the sauce is bubbling and the top is golden-brown.
- Let the gratin rest for 5 minutes before garnishing with fresh parsley and serving hot.