Heat oven to 300°F (150°C). Coarsely chop tomatoes with their juice in a food processor or go at them in their can with kitchen shears to break them up.
Put roast in an ovenproof 4- to 5-quart heavy pot or a casserole dish with a lid — a tight fit is just fine, as it will shrink very soon.
Pour tomatoes over roast and scatter garlic around it. Season generously with salt and pepper. Braise in middle of oven, covered, until very tender, 3 to 4 hours. Cut roast into 1/4-inch-thick slices and serve with sauce, garlic, squeezed from its peels.
Like almost all braises, this is even better on the second day, which means it’s a dream of a dinner party dish. Let it cool to room temperature, then chill it in the fridge until needed. De-fat the sauce before warming and rewarm at 300 for 30 to 45 minutes.