Tip the lamb into a large bowl. Peel and halve the onion. Finely chop one half and add it to the lamb. Finely slice the other half and set aside.
Peel and crush the garlic. Grate or pare the zest from the lemon. Finely chop half the mint leaves. Add the garlic, lemon zest and mint to the lamb.
Season the meatball mixture. Mix everything together with your hands. Shape into 10 balls. Put them on a plate. Cover with cling film. Bung in the freezer for 5-10 mins to chill.
Tip the bulgar wheat into a heatproof bowl. Pour in 250ml boiling water. Cover with cling film or a plate. Leave for 20 mins till the water is absorbed and the bulgar wheat is tender.
Coarsely grate the cucumber. Pop it into a colander or sieve. Press down to squeeze out as much liquid as possible. Tip the cucumber into a bowl.
Finely chop the rest of the mint leaves. Add them to the cucumber with the yogurt. Season with pepper and stir together.
Heat ½ tbsp oil in a frying pan over a medium heat. Fry the meatballs for around 10 mins till they’re cooked through. Turn every so often so they brown evenly.
Arrange the watercress on plates or a big platter. Scatter over the sliced onion. Halve the lemon. Juice one half. Slice the other half into wedges. Whisk the lemon juice with 1 tbsp oil. Season. Drizzle over the salad. Serve with the meatballs, bulgar wheat, yogurt and the lemon wedges for squeezing.