Grilled mackerel with beetroot and raspberry couscous

Ingredients

• 4 mackerel, heads removed, cleaned, about 200 g (7 oz) each
• Little salt and freshly ground black pepper
• 200 g (7 oz) couscous
• 150 g (5 oz) chilled cooked beetroot in natural juices, drained, diced
• 400 ml (14 fl oz) boiling water
• Grated rind and juice of 1 lemon
• 3 tablespoons olive oil
• 1 small red onion, finely chopped
• 150 g (5 oz) fresh raspberries
• Small bunch fresh parsley or a mix of parsley and mint, roughly chopped
• Lemon wedges, crusty bread or toasted pitas to serve, optional