Step 1Finely chop a quarter of the onion and half of the coriander. Mix in a bowl with the avocado and chopped tomato. Set aside.
Step 2Meanwhile, spray beef with olive oil. Combine the cumin, coriander, paprika and oregano in a small bowl, and sprinkle evenly over both sides of beef.
Step 3Heat a barbecue hotplate to medium-high. Cut remaining onion into thin wedges. Grill the onion, mushrooms and capsicums for 7–8 minutes, or until charred and tender. Transfer to a medium bowl and cover to keep warm.
Step 4Grill beef for 3–4 minutes each side, or until cooked to your liking. Transfer to a small plate. Cover with foil for 5 minutes to rest before slicing thinly.
Step 5Meanwhile, heat tortillas on a hot pan until warmed through. Serve tortillas topped with the capsicum mixture, beef, guacamole, remaining coriander sprigs and lime.