Remove the sirloin from the packaging and place in a shallow bowl of pie plate. Mix the marinade ingredients and pour over the sirloin, being sure to coat both sides of the steak. Leave to marinade for at least 15 minutes but up to 24 hours.
While the steak is marinating, slice the onions and both peppers into long thin strips. Set aside.
Heat a nonstick or a cast iron skillet over high heat. Remove the steak from the marinade and sear in the hot pan for about 4-5 minutes on each side. Remove the steak and leave to rest.
Place the onions in the pan and cook until soft and starting to turn golden, about 5 minutes. Add the peppers to the pan and cook for a further 10 minutes. Pour in the marinade and heat until it boils. Let it simmer until it thickens into a sauce, which should take about 3 minutes. Then, take it off the heat.
Slice the steak width wise, across the grain of the meat, and place in the pan with the peppers and onions.
Top the tortillas with a tablespoon of sour cream or Greek yoghurt and the steak and peppers. Sprinkle with the coriander and serve with extra lime wedges and avocado.