Prep Meatballs: Combine all meatball ingredients (except oil) in a bowl, until well combined. Use a tablespoon measure and damp hands to shape mixture into balls.
Cook Meatballs: Heat oil in a frypan on medium heat. Cook meatballs (in batches if needed) for about 10 minutes, turning frequently, until cooked through. Rest, covered, for 2-3 minutes.
Make Pickled Onion: Thinly slice red onion and toss in a small bowl with vinegar, honey and a pinch of salt. Set aside.
Make Tzatziki: Grate first measure of cucumber, squeeze all liquid out and roughly chop dill. Mix in a second bowl with yoghurt and season to taste with salt. Cut remaining cucumber in half and thinly slice. Set aside.
Prep Salad: Separate lettuce leaves and thinly slice avocado. Toss in a salad bowl with sliced cucumber, cherry tomatoes, vinegar and olive oil.
Serve: Divide salad between plates and top with meatballs. Sprinkle over pickled onion and walnuts. Serve tzatziki on the side