For the meatballs: Put the beef, breadcrumbs, barbecue seasoning, ground coriander, ground cumin, garlic, onions, bell peppers, chopped coriander and some salt and pepper into a large bowl and mix well to combine. Take 1 tablespoon of the mix and roll it in your hands to form a ball. Repeat with rest of the mixture.
Heat some oil in a large frying pan. Add the meatballs and fry until golden brown on all sides. Put on a lid or cover with tin foil and cook for another couple minutes until cooked through.
For the tzatziki: Slice the cucumber in half lengthwise and scrape out the core with a spoon. Grate the cucumber and then squeeze out any excess moisture before transferring to a bowl. Add the yoghurt, sour cream if using and mint and stir to combine. Season with salt and pepper.
To serve, toast the pittas. Cut in half and fill with the meatballs, rocket, tomatoes and tzatziki.
To make your own breadcrumbs, put 4 slices brown bread (whole wheat bread), crusts removed, into the oven at 200 degrees C (390 degrees F) for 5 minutes, then blitz in a food processor.