Place oil, lime juice, cumin, salt, and pepper into a medium bowl, whisking to combine. Place beef, onions, and garlic into a large zip top bag and pour marinade inside bag. Close top of bag and with hands, massage beef with marinade to coat evenly. Place in refrigerator and marinade for 1 hour.
Heat oil in grill pan over medium heat. Grill beef until browned, about 3 minutes per side. Transfer to a cutting board to rest for 5 minutes. Cut thin slices against the grain or on the bias.
Place avocado, lime juice, salt, and pepper into a medium bowl. Mash with a fork until well combined.
Place corn and peppers into a medium bowl, mixing to combine.
Place beef slices, a dollop of guacamole, and a spoon of corn and peppers into soft tortilla bread. Serve warm.