Step 1Heat the oven to 350 degrees. Place the meat fat-side up in a large Dutch oven. Sprinkle the onion soup mix over the meat. Cover with the chile sauce and 2 cups of water, or more if needed to almost cover the meat. Crush the garlic cloves and add to the liquid.
Step 2Cover the pan and cook for 4 hours. Let the brisket cool for about 45 minutes and refrigerate overnight. Then skim the fat off the meat.
Step 3About 1 1/2 hours before you wish to serve the brisket, heat the oven to 350 degrees. Remove the brisket to a cutting board and slice it thinly across the grain. Trim, peel and cut the carrots into one-half-inch-by-2-inch sticks. Cook the brisket and carrots covered for 1 hour, until the brisket is heated through and the carrots are fork tender. (Alternatively, the brisket can be completed the same day: While the brisket is cooling for 45 minutes, trim, peel and cut the carrots into one-half-inch-by-2-inch sticks. Remove the brisket and slice it thinly across the grain. Skim the fat off the top of the liquid, add the sliced brisket back to the pan with the carrots, cover and bake at 350 degrees for 1 more hour, or until carrots are fork tender.) Serve on a platter.