Heat oven to 350 degrees F. Put flour in cooking bag and spread the flour to allow it reach all parts of bag. Put bag in a large roasting pot and spread the flour equally over the bottom using your fingers.
Sprinkle the onion powder, oregano, garlic powder, salt, and pepper over the brisket and season both the sides of the brisket. Place brisket, plump-side up, in the cooking bag on top of the flour.
Garnish potatoes, mushrooms, carrots, and whole garlic cloves evenly around the outside of the brisket within the bag. Place the finely-sliced onions above the brisket.
In a bowl mix tomato juice, onion soup mix, gravy mix, and water and form a smooth puree. Pour this mixture on top of brisket and vegetables. You can also use cooking spray to lightly spray the tops of the vegetables to prohibit them from discoloring. Seal the bag with the plastic tie. Make 4 cuts of 1-inch each in the top of the bag for ventilation.
Bake at 350 degrees F for 3 hours. Leave it for 15 minutes prior to slicing brisket. Slice brisket against the grain into fine slices. Serve with natural gravy and other vegetables.