In a 10- to 12-inch nonstick frying pan over medium-high heat, stir onion, garlic, and cumin seed until onion is limp, about 5 minutes. Add enchilada sauce and stir until simmering.
Discard any solidified fat from beef and sauce. Scrape sauce from meat and stir into enchilada sauce mixture. Cut beef into 3/4-inch cubes and add to pan; cover, reduce heat to medium-low, and simmer, stirring occasionally, until beef is hot, 5 to 8 minutes.
Spoon chili into bowls. Serve with sour cream and green onions to add to taste.