In a mixing jug, combine all the marinade ingredients and whisk until incorporated. Place the steak in a shallow dish, so that it fits snuggly. Pour the marinade over the beef and set aside to marinate for at least 30 minutes.
While the steak is marinating, slice and cook the vegetables. Place the oil in a non stick skillet and heat on high. Add the peppers and onion and cook until the vegetables are tender, but still have a slight crunch. Place in the oven to keep warm while you sear the beef.
Heat a grill pan over very high heat. Toast the flour tortillas in the grill pan for about a minute each side. Once the tortillas are toasted, place the steak in the pan and grill on each side for about 5 minutes, or until each side is charred and smokey. Set aside to rest for 10 minutes before slicing.
Place the veggies and the sliced beef in a bowl to serve at the table along side the tortillas, sliced chillies, avocado, and cilantro.