Whisk marinade ingredients together to combine. Pour out half of the marinade into a shallow dish to marinade the steak for an hour at room temperature (or in the refrigerator for at least 2-4 hours or overnight. Remove from the refrigerator 30 minutes prior to cooking). Refrigerate the reserved untouched marinade to use later.
Heat about one teaspoon of oil in a grill pan or cast iron skillet over medium-high heat and grill steak on each side until desired doneness (about 4 minutes each side for medium-rare, depending on thickness). Set aside and tent with foil to allow to rest for 5 minutes.
Wipe pan over the paper towels; drizzle with another teaspoon of oil and fry peppers (capsicums) and onion strips. Add in half of the reserved marinade, salt and pepper, and continue frying until cooked to your liking.
Slice steak again the grain into thin strips.
Serve with warmed tortillas, extra cilantro, sour cream, sliced avocado ( or your desired fillings), and drizzle over the remaining reserved marinade.