Poke holes all over meat and insert slivers of garlic. Place in a nonreactive pan (a glass baking dish or large Ziploc bag will work, too.) Combine rosemary and vinegar, pour over meat, turning to coat. Refrigerate overnight.

When ready to cook, heat oven to 350 degrees. Spread onions in a large casserole or roasting pan. Nestle meat into onions and pour marinade over all. Add tomatoes and juice over the top. (No need to stir.)

Cover tightly with lid or foil and cook until easily pierced by fork, three to three and a half hours. Remove lid and allow to rest for 10 minutes. Defat juices if desired and pass with roast.

If you wish, you may slightly thicken juices with a teaspoon to a teaspoon and a half of cornstarch mixed with a bit of water. Stir into juices, bring to a simmer and serve.