Cook and stir beef in a stockpot over medium heat until browned, 5 to 7 minutes. Add onions and garlic; cook and stir until onions are softened, 5 to 10 minutes. Stir harissa into beef mixture until coated; cook and stir for 2 minutes more. Season beef mixture with salt and pepper.
Pour enough water over beef to completely cover ingredients and bring to a boil. Add potatoes and tomatoes to stew; reduce heat to medium-low and simmer until beef and potatoes are tender, 2 to 3 hours.