How to Make It
Preheat the oven to 400°F. In a large skillet, melt 2 tablespoons of the butter over medium heat then toss with the garlic, breadcrumbs and a pinch each of salt and pepper. Set the bread crumbs aside. Thinly slice the Swiss chard stems. Coarsely chop the Swiss chard leaves.
In the same skillet, heat 2 tablespoons of the butter over medium heat until melted, then stir in the onion and Swiss chard stems and cook, stirring occasionally until softened, about 8 minutes. Stir in the Swiss chard leaves and cook, turning with tongs, until the leaves are wilted and all the liquid in the skillet has evaporated, about 6 minutes. Remove the skillet from the heat.
In a small saucepan, melt the remaining 1 tablespoon butter over medium heat until melted, then whisk in the flour and cook, stirring, 1 minutes. Whisk in the stock, milk and cream and bring to a boil, whisking. Remove from the heat and whisk in the cheese. Stir the sauce into the Swiss chard mixture, then transfer to a buttered 2-quart baking dish. Sprinkle the breadcrumbs evenly over the top of the casserole. Bake the casserole until the breadcrumbs are golden and the Swiss chard is bubbling, 25 to 30 minutes. Let cool slightly then serve.