In a Dutch oven, cook the beef and onion over medium-heat until no longer pink; spoon out any accumulated fat and discard. Stir in the chili powder, flour, salt and garlic powder until blended; gradually stir in the water.
Add the beans. Bring to a boil. Reduce the heat; simmer, uncovered, for 12-15 minutes or until thickened, stirring occasionally.
To serve, divide the chips among six serving bowls. Top with the beef mixture, lettuce, cheese and tomatoes. Garnish with sour cream and cilantro, if desired.