5 1/2 pounds mixed bitter greens, such as Swiss chard, escarole, turnip or
mustard greens, chicory, broccoli rabe, kale and spinach, tough stems
1 stick (4 ounces) unsalted butter
4 large leeks, white and tender green, thinly sliced crosswise
2 garlic cloves, thinly sliced
3 cups chicken stock, fresh or canned low-sodium broth
Freshly ground pepper
3 pounds Yukon Gold potatoes
Bring a large pot of water to a boil and add salt. Cut the greens into 1-inch pieces and wash them thoroughly. Add the greens to the pot and cover. As soon as the water returns to a boil, drain the greens in a colander; pat dry.
Preheat the oven to 375°. In a large skillet, melt 4 tablespoons of the butter. Add the leeks and garlic and cook over moderate heat until just beginning to brown, about 10 minutes. Add the Chicken Stock and bring to a boil. Season generously with salt and pepper and remove from the heat.
Peel and halve the potatoes and slice them 1/4 inch thick. In a large bowl, toss the potatoes with the greens and leeks. Season with salt and pepper. Transfer the mixture to a large nonreactive baking dish and press the vegetables down firmly. Dot with the remaining 4 tablespoons of butter. Cover the pan tightly with foil and bake for 1 hour. Uncover and bake for about 15 minutes longer, or until the potatoes are very tender.
This dish can be prepared early in the day and reheated in the oven.