Heat oil in a large Dutch oven over medium-high heat; add onions. Cook, stirring often, 25 to 30 minutes or until onions are golden brown and caramelized. Stir in 1 teaspoon garlic salt and 1/2 teaspoon pepper. Remove onions from pan with a slotted spoon, reserving oil in pan.
Sprinkle flour, paprika, and remaining 1/2 teaspoon each garlic salt and pepper on all sides of brisket. Heat reserved oil in Dutch oven over medium-high heat. Sear brisket 2 to 3 minutes on each side or until lightly browned.
Combine beef bouillon cube and 1/2 cup water, stirring until bouillon is dissolved. Pour over brisket halves. Add onions and tomato sauce; bring to a boil. Cover, reduce heat, and simmer 2 1/2 hours.
Remove brisket from Dutch oven, and place on a large, rimmed baking sheet or platter. Place another baking sheet on top of brisket; place a heavy skillet or large cans on top of baking sheet. (Weighing down the cooked brisket improves texture and makes slicing easier the next day.) Refrigerate brisket overnight. In a separate container, cover and refrigerate onions and pan juices.
Skim fat from onion-tomato mixture; discard fat. Heat mixture in Dutch oven over medium-low heat. Slice brisket across grain into thin slices. Add brisket to pan, and cook 10 minutes or until thoroughly heated.
Note: The grain in brisket often changes while slicing, so continually reposition the brisket and slice against the grain.