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Cook green beans in a large pot of boiling salted water until crisp-tender, about 4 minutes.
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Drain.
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Transfer to a bowl of ice water and cool. Drain again and pat dry.
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Melt butter in a large deep skillet over medium heat.
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Add walnuts; sauté until nuts are crisp and butter is lightly browned, about 3 minutes.
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Add beans and toss to heat through, about 5 minutes.
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Mix in lemon peel and toss 1 minute.
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Mix in parsley.
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Season beans to taste with salt and pepper.
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Transfer to a bowl and serve.