In a 12-inch nonstick frying pan or a 5- to 6-quart pan over medium-high heat, frequently stir onion in oil until limp, 4 to 5 minutes.
Add cumin, coriander, cayenne, and turmeric and stir until fragrant, about 30 seconds. Add tomatoes and their juice; bring to a boil over high heat, stirring often.
Discard any solidified fat from beef and sauce. Scrape sauce from meat into pan. Cut beef into 3/4-inch cubes and add to pan. Cover, reduce heat to low, and simmer until meat is hot, 5 to 7 minutes.
Meanwhile, in a 2- to 3-quart pan over high heat, bring broth to a boil. Stir in couscous, cover pan, and remove from heat; let stand until broth is absorbed and couscous is tender to bite, about 5 minutes. Pour couscous into a wide bowl. Spoon stew over couscous and sprinkle with mint.