1. Coat beef with cooking spray. Cook beef in a large Dutch oven coated with cooking spray over high heat 3 minutes; stir in onion. Cook 5 minutes or until liquid evaporates and beef and onion are browned.
2. Stir in 3 cups water, tomatoes, and cumin; cover and bring to a boil. Reduce heat to medium; simmer 20 minutes. Add potato; cover and simmer 10 minutes or until potato is tender. Remove from heat; stir in cilantro, if desired.