Turkish tuna kebab

Name: Turkish tuna kebab
Ingredients: 2 tbsp rapeseed oil; 1 tsp harissa paste; 2 tsp ground cumin; 1 tsp smoked paprika; 1⁄2 tsp garlic granules; 4 tuna steaks (2 x 240g or 220g packs), cut into chunky cubes; 4 red onions, cut into wedges; 1 tbsp rapeseed oil; 1 tsp sumac; handful of flat-leaf parsley, leaves picked; 100g Greek yogurt; juice of 1⁄2 lemon; 1 tsp garlic paste (or 3-4 garlic cloves, grated); 1 x 200g pack Persian flatbreads (we used Deli Kitchen); 1 romaine lettuce, shredded; 100g roasted red peppers, sliced; pickled chillies (optional)