Preheat barbecue to high. To make green olive salsa verde, combine mint, parsley, olives, garlic, anchovies, almonds, and capers in a bowl and toss to mix well. Add oil, zest and juice, and gently whisk to combine. Season with salt and freshly ground black pepper to taste.
Drizzle 2 tsp oil over prawns, season, and toss to coat. Cook for 2 mins each side on the grill or until lightly charred and just cooked. Transfer to a plate and cover to keep warm.
Brush onions, witlof, and haloumi with remaining 2 tsp oil. Season onions and witlof, then cook on the grill for 2 mins each side or until lightly charred. Transfer to a plate. Cook haloumi on the grill for 1 min each side or until lightly charred.
Arrange onions, witlof, haloumi, prawns and watercress on a platter. Drizzle with salsa verde and serve immediately.
10 / 11 / 2016