Mix jalapenos, parsley, capers, mayonnaise and mustard in a bowl. Cover and leave in the fridge to chill

Bring barbecue to medium-high heat

Drizzle barramundi and cos hearts with olive oil, scatter with lemon rind and season with salt and pepper to taste

Grill barramundi, turning once, until browned and cooked through, approx. 3-4 minutes each side, and rest briefly. Grill lettuce and lemon halves cut-side down until charred, approx. 1-2 minutes

To serve, grill cut sides of buns before generously spreading bases with jalapeno tartare. Top with barramundi, a lettuce heart and season to taste. Add bun tops and serve with charred lemon