Place one rack in the center of the oven and another in the bottom. Heat oven to 500 degrees F.
Place brisket, fat side down, in a 14 x12 x 2-inch roasting pan. It's best not to use a shallow pan for this, or later the juices might run over and out. Put the garlic in the pan. Drizzle with 1 teaspoon of the oil. Using your hands, rub oil onto garlic.
Put the onion wedges and carrots into a smaller, 12 x 8 x 1 1/2-inch roasting pan. Drizzle with remaining oil. Using your hands, rub oil over all the pieces until they are well coated. Arrange onion wedges on their sides, so that they touch carrots as little as possible.
Roast vegetables on center rack of oven for 30 minutes. Turn with a spatula. Roast 15 minutes more. Roast the brisket simultaneously on the bottom rack of the oven for 15 minutes. Turn over so fat side is up. Roast 30 minutes more.
Remove both pans from oven. Lower heat to 275 degrees F, leaving the door ajar a minute or two to reduce temperature more quickly. Leave the brisket and garlic in the large pan and add 1/4 cup of the water. The water should bubble up from the heat of the pan; if it does not, put pan on the stove over medium-high heat. With a wooden spoon, scrape around the brisket to scoop up and remove the brown bits from the bottom of pan.
Put the onions and carrots into the brisket pan. Put the smaller pan on top of the stove. Add the remaining water and bring contents to a boil while scraping the bottom vigorously with a wooden spoon. Pour liquid into brisket pan. Add roasted plum tomatoes and reserved roasting liquid or, if using canned tomatoes, stick a small knife into the opened can of tomatoes. Run it back and forth through the tomatoes and puree until only small pieces remain. Pour contents over vegetables and around brisket. Put the bay leaf into the liquid. Cover tightly with foil.
Roast on center rack of oven for 3 1/2 hours. Remove foil, increase temperature to 500 degrees F, and roast 15 minutes more or until brisket is nicely browned.
Remove. Sprinkle with salt and pepper. With the back of a spoon, smush the softened garlic out of each section and into the liquid. Throw the empty papers away. Transfer brisket to a platter and cover brisket with a tent of foil. Use slotted spoon to transfer vegetables to a large serving bowl or plate. Discard the bay leaf. Pour remaining liquid into a measuring cup. Let sit long enough for the fat to rise to the surface, about 15 minutes. Skim fat off surface.
To serve, cut brisket against the grain into long thin strips. Arrange 2 to 3 slices on each plate. Spoon on some of the vegetables, and moisten everything with a generous amount of the liquid.