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Preheat oven to 325 degrees.
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Arrange cabbage wedges in a 9 x 13-inch baking dish (ideally, not overlapping too much). Tuck spring onions all around. Grate zest over top. Pour in broth and vinegar, drizzle all over with olive oil, and sprinkle with spices. Cover tightly with foil and bake for 1 hour.
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At the hour mark, carefully flip the wedges (they may fall apart or collapse), and add a couple tablespoons water as needed. Return to oven and cook one hour longer, or until cabbage is very tender.
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Turn heat up to 400, uncover the cabbage, and roast 15 minutes, or until the edges have crisped and the cabbage is a deep golden hue.
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To serve, top with flaky sea salt and reserved spring onion greens.