Barbecued Greek lamb and beef meatballs

350g lean beef, finely minced

350g lean lamb, finely minced

1 large onion, finely chopped

4 tbsp flat-leaf parsley, finely chopped

2 tbsp dried oregano (rigani), briefly pounded in a small mortar

1 tsp dried thyme, crumbled

75g fresh wholemeal breadcrumbs

1 tbsp mustard seeds or ½ teaspoon powdered mustard

4 tbsp dry red wine or 2 tbsp aged red wine vinegar

coarse grain sea salt

cracked black pepper

extra virgin olive oil

For serving:

2 small red or mild onions, quartered and thinly sliced

1 small bunch of flat-leaf parsley, leaves coarsely chopped

3 ripe tomatoes, skinned and cut into small dice

½ medium cucumber, peeled and cut into small dice

juice of ½ lemon

1 tbsp sumac or 1 tsp paprika and a large pinch of cayenne pepper

watercress sprigs

lemon wedges