Combine all the ingredients (except the steak) in a blender and process until smooth.
Place the steak in a large bowl and pour in the marinade. Ensure the steak is well coated. Cover the bowl and refrigerate for at least 1 hour, or up to 3 hours.
Heat a grill, griddle or heavy skillet over a high heat. Cook the steak, in batches if necessary, turning it once, until both sides are browned and steak is cooked to your liking.
Let the steaks rest for about 3 minutes, and then cut them against the grain into slices.
Place the steak in the warmed tortillas, top with onion, coriander and chillies, and serve with lime wedges.
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