Make the spice mix: Combine the cumin, chilli powder, paprika, salt and pepper in a small bowl. Add half the mix to the beef strips and toss to coat. Add the other half to the sliced peppers and toss separately.
Sear: Heat a wok over high heat and add the olive oil. When smoking hot, tip in the peppers and sauté until slightly soft. Toss in the beef strips and cook for 2 minutes, stirring and tossing frequently, until just cooked. Remove from the wok and keep warm.
Warm the tortillas: Heat each tortilla in a dry frying pan for about 10 seconds on each side.
Assemble: Place some beef and peppers along the centre of each tortilla. Add a dollop of soured cream and some guacamole. Scatter over torn coriander and roll up. Serve immediately.
