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1
Preheat oven to 300F.
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2
Cut onions into 1/4-inch-thick rings.
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3
Quarter tomatoes lengthwise.
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4
Tie each shank securely with kitchen string to keep meat attached to bone and season with salt and pepper.
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5
In an ovenproof 5- to 6-quart heavy kettle just wide enough to hold shanks in one layer toss together onions and tomatoes and season with salt and pepper.
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6
Add shanks and braise, covered, in middle of oven until very tender, about 3 hours.
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7
Peel potatoes and cut into roughly 2-inch pieces.
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8
In a 4-quart kettle cover potatoes with cold salted water by 2 inches and simmer until tender, about 25 minutes.
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9
Reserve 1/2 cup potato cooking water and drain potatoes in a colander.
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10
Return potatoes to kettle and add butter and 1/4 cup reserved cooking water.
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11
Mash potato mixture with a potato masher until smooth, adding enough of remaining cooking water to reach the desired consistency, and season with salt and pepper.
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12
Shanks and potatoes may be made up to this point 2 days ahead and cooled, uncovered, before being chilled separately, covered.
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13
Reheat shanks and potatoes before proceeding.
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14
Transfer shanks, onions, and tomatoes to a platter with a slotted spoon and keep warm, loosely covered.
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15
Boil braising liquid until reduced to about 1 cup, about 5 minutes.
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16
Season jus with salt and pepper.
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17
Serve shanks, onions, and tomatoes with mashed potatoes and jus.
