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Trim the outer leaves of the Brussels sprouts and cut an X in the bottom of each.
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In a large pot of boiling salted water, cover and cook the Brussels sprouts until tender-crisp, about 6 minutes.
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Drain and chill under cold water, then drain well.
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In a large nonstick skillet, toast the mustard seeds over medium heat, stirring, until aromatic and the seeds begin to pop, about 3 minutes.
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Add the oil, onion, garlic, salt and pepper.
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Cook over medium-high heat until softened, about 2 minutes.
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Add the Brussels sprouts, stock, mustard and lemon juice and cook, tossing occasionally, until heated through and the Brussels sprouts are coated, about 2 minutes.