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1
Put oven rack in middle position and preheat oven to 350u00b0F.
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2
Pat shanks dry and rub all over with salt and pepper.
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3
Dredge shanks in flour, turning to coat. Heat 1 1/2 tablespoons oil in a 12-inch heavy skillet over moderate heat until hot but not smoking, then brown shanks on all sides in 3 batches, adding 1/2 tablespoon oil if necessary, 5 to 6 minutes per batch.
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4
Transfer shanks to a large roasting pan.
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5
Add broth and water to skillet and bring to a boil, scraping up brown bits.
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6
Nestle garlic (cut sides down), zest, thyme, and bay leaves with celery and carrots around shanks in pan and add broth mixture. Cover surface with a sheet of parchment paper and tightly cover pan with foil. Transfer to oven and braise until meat is very tender, 2 1/2 to 3 hours.
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7
Skim off and discard fat from cooking liquid and cool mixture completely, uncovered, about 1 hour, then chill, covered, at least 6 hours.
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8
Preheat oven to 350u00b0F (leave rack in middle position).
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9
Skim off and discard any remaining fat from surface of pan juices and reheat shank mixture, covered, in oven, turning shanks over once, 1 hour.
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10
Discard bay leaf, thyme sprigs, and zest. Transfer shanks and vegetables with garlic to a serving dish and keep warm, covered.
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11
If pan juices measure more than 3 cups, boil in a cleaned 12-inch skillet until reduced; if less, add water. Squeeze garlic pulp from 1 head into pan juices, discarding skin, and whisk to incorporate.
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12
Add lemon juice and salt and pepper to taste.
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13
Pour sauce over shanks and serve, along with vegetables and remaining garlic heads.
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14
Cooks' note: Braised shank mixture can be chilled, covered, up to 2 days.
