Trim the excess fat from the brisket. Poke a lot of holes all over brisket with a skewer. Briskets are usually long, so cut meat in half so that it will fit into a large, resealable bag with the marinade.
In a blender, combine garlic, cilantro, and vinegar. Blend until smooth. Pour marinade into resealable bag with the meat. Marinate in the refrigerator for 24 hours.
Preheat oven to 300 degrees F (150 degrees C).
Shake flour into largest-size oven cooking bag. Place brisket into bag. Close bag and pierce as instructed on package. Layer the onions on top of the brisket, inside of the bag. Place on baking pan in the center of the oven.
Bake in preheated oven until tender, about 6 hours. Slice thinly across the grain to serve.
