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To make the topping: Set the oven rack in the center and preheat the oven to 225°F.
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Combine the smashed garlic cloves with the potatoes, 1/2 teaspoon salt, rosemary, and whole garlic cloves in a deep, oven-proof dish, and pour in olive oil to cover.
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Bake for 1 1/2 hours, then remove from the oven and let potatoes cool to room temperature in the oil. Discard the garlic cloves and rosemary sprigs. Use a slotted spoon to remove the cooled potatoes from the oil; reserve the oil.
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To make the filling: Season the steak cubes with 1 teaspoon salt and 1/2 teaspoon pepper. Heat 1 tablespoon of the oil in a large, heavy skillet until almost smoking, and brown the steak in a single layer, in batches. Once all the steak is nicely browned, set it aside.
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Heat the remaining oil in a large Dutch oven. Add the leeks, garlic, and 1/2 teaspoon salt and sweat (cook slowly) for 5 minutes. Add the steak and chicken stock. Cook, uncovered, stirring occasionally, over medium-low heat for 30 minutes, or until most of the liquid has evaporated.
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Add the cabbage, and remaining 1/2 teaspoon salt and 1/2 teaspoon pepper. Cook, covered, for another 10 minutes, or until the cabbage is soft but not completely wilted. Stir in the cheese, making sure it melts completely.
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To serve: Transfer the filling to a casserole serving dish and keep it warm while you heat the reserved olive oil to 350°F in a 2-quart saucepan. Drop the cooked potatoes into the oil and fry until golden brown, about 3 minutes. Drain on paper towels and season with the remaining 1/2 teaspoon salt before spooning over the top of the casserole just before serving.
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Refrigerate any leftovers for several days; freeze, well wrapped, for longer storage.