1) Combine the parsley, coriander, garlic, peppers and oil in a food processor and process until smooth. Transfer the mixture to a large baking dish, add the beef and toss to coat in the mixture. Cover and refrigerate for at least 30 mins and up to 8 hrs.
2) Heat the Barbeque to high. Skewer 2 pieces of the beef onto 2 skewers so that the meat lays flat on the griddle. Season the meat on both sides with salt and pepper and barbecue until golden brown, slightly charred on both sides and cooked to medium-rare doneness, about 3 mins per side.
3) Brush the spring onions with canola oil and season with salt and pepper. Barbecue for 2 to 3 minutes per side or until slightly charred and just cooked through.
4) Serve skewers with griddled spring onions and the sherry vinegar steak sauce.
For the sherry vinegar steak sauce:
Place the peppers, vinegar, mustard, horseradish, honey, treacle, Worcestershire, salt and pepper in a food processor or blender and blend until smooth. Can be made 1 day in advance and refrigerated.