In a small bowl, whisk together marinade ingredients.
Place steak in a shallow container, and pour marinade over it. Refrigerate, covered, 8 hours or overnight to tenderize meat.
Drain the marinade from the meat. Grill the steak on a grill pan over medium-high heat for 4 minutes each side until just slightly pink in the center. Transfer the steak to a cutting board and let rest for at least 15 minutes.
Meanwhile, in the same pan with the steak juices, add the bell peppers and onions. Sauté the mixture for 5-7 minutes until the vegetables are slightly softened. Thinly slice the steak against the grain on a diagonal and add to the veggies and toss to combine. Serve with warm tortillas, salsa and guacamole!