8 h 20 m
Place brisket fat side up into slow cooker, sprinkle garlic powder on top and rub ketchup generously into brisket. Add onion slices all around and on top of meat, add enough water to coat the bottom of the cooker.
Cook on Low, all day, for the most tender brisket you will ever eat.
For gravy, remove brisket from the slow cooker, and increase temperature to High. Mix flour with 1/4 cup water, add to pot, and bring to a boil, stirring constantly.