Whirl garlic and ginger in a food processor until minced. Trim fat from steaks and cut across the grain into 1/4-in. slices, discarding pockets of fat. Sprinkle meat with half of garlic mixture and all the salt.
Heat oil in a 5- to 6-qt. pan over high heat. Brown half the beef lightly, stirring occasionally, 3 to 6 minutes. With a slotted spoon, transfer meat to a plate. Repeat with remaining beef.
Reduce heat to medium. Add remaining garlic mixture to pan and cook until softened, about 30 seconds. Stir in five-spice powder, then broth and soy sauce. Peel sweet potato, halve lengthwise, and cut in 1/3-in.-thick slices. Add to broth. Cover and bring to a boil over high heat, then reduce heat and simmer 2 minutes.
Meanwhile, trim ends from bok choy, rinse, separate leaves, and cut in wide diagonal slices. Stir bok choy into stew and simmer just until wilted, about 2 minutes. Stir in beef and cook until hot, 1 to 2 minutes. Cut onions in 3-in. slivers.
Ladle stew into bowls and garnish with onions.
Note: Nutritional analysis is per serving.