Heat oil in a cast-iron skillet over medium-high heat; saute onion until lightly browned, 5 to 10 minutes. Add ground beef; cook and stir until beef is browned and crumbly, 5 to 10 minutes. Mix pinto beans and black beans into beef mixture.
3
Sprinkle Mexican cheese blend over beef-bean mixture; stir. Mix diced tomatoes with green chile peppers into beef-bean mixture.
