How to Make It
In a large pot of boiling water, cook the chard for 2 minutes; drain. When the leaves are cool enough to handle, squeeze out the excess water. Coarsely chop the chard.
In the same pot, heat the olive oil. Add the onions and thyme and cook over moderately low heat, stirring occasionally, until softened, about 12 minutes. Add the chard and the wine and simmer over moderately high heat until the wine is reduced to 1/4 cup, about 5 minutes. Season with salt and pepper.
Preheat the oven to 400°. In a small saucepan, bring the stock to a simmer. Butter a 10-by-15-inch baking dish. Line the bottom of the dish with one-third of the bread, overlapping the slices slightly and cutting the bread to fit. Top with half of the tomato slices and season with salt and pepper. Spread half of the chard on top, then sprinkle with half of the cheese. Repeat the layering once and finish with the remaining bread. Pour the hot stock over the casserole and press with a spatula. Brush the top with the melted butter.
Cover the dish with foil and bake in the upper third of the oven for 1 hour. Uncover the dish and bake for about 10 minutes longer, until the top is browned and crisp. Let the casserole rest for at least 10 minutes before serving.