Trim excess fat from the brisket if needed, or ask the butcher to trim it for you. Leave a thin layer of fat on top. Place in an 8×8-inch baking dish.
Pour soy sauce onto both sides of brisket and spread over all surfaces. Cover with foil and place in the refrigerator to “brine” at least 1 hour, or up to 12 hours.
To prepare and cook:
Preheat oven to 300°F.
Mix sugar, oregano, paprika, chili powder, salt, pepper, and red pepper flakes in a small bowl. Rub half of the spice mixture onto the top of the brisket (side with the layer of fat). Return meat to baking dish with the top, spiced side down.
Coarsely chop garlic and combine with vinegar, water, and Worcestershire sauce in a liquid measuring cup. Pour over the brisket.
Sprinkle remaining half of dry rub over top of brisket. Cover with foil and cook 3 hours.
Optional step: Remove foil and turn oven to broil. Broil top just long enough to become brown and crisp, but not burned, about 1 or 2 minutes.
Keep covered with foil and let stand at room temperature 10 to 30 minutes. Transfer brisket to a cutting board, and slice into 1/4 to 1/2-inch-thick pieces, cutting against the grain.
Optional Sauce:
At this point, you can chill the juices about 1 hour, then remove the solid fat from the top.
Place the brisket slices into the defatted juices and let stand in them up to 24 hours. This makes the meat even more flavorful.