In a large shallow dish or zip-top bag, combine garlic, paprika, chilli powder, oregano, 2 tbsp oil, 2 tbsp lemon juice, lemon zest, and 1/4 tsp each salt and pepper. Cut steak into 3cm cubes, add to marinade, and set aside to until ready to cook.
Place olives and parsley in a food processor. Pulse to chop, then add 2 tbsp olive oil and 2 tbsp lemon juice. Pulse again to form a rough sauce.
Steam potatoes and cauliflower 10 - 15 mins, until tender when pierced with a fork. Remove from heat and carefully tip vegetables into a large bowl to let steam evaporate.
Whisk together vinegar, ¼ cup oil, mustard, and honey. Season with salt and pepper. Gently stir half the dressing through the warm vegetables, let cool.
Preheat BBQ or a heavy-based skillet to medium-high. Drain excess marinade from beef cubes and thread onto skewers. Cook 8 - 10 mins, turning regularly, or until cooked to your liking. Transfer to a plate, cover and rest.
Add celery, celery leaves, and parsley to bowl with potatoes and cauliflower. Add remaining dressing to taste, and adjust seasoning if necessary.
Serve Portuguese beef skewers with parsley sauce and potato cauliflower salad.
You will need bamboo or metal skewers for this recipe – soak bamboo skewers in water for 5-10 minutes before using to prevent burning.