To make the prawn stock, peel the prawns and remove the heads. Place the prawn shells and heads in a large pot then add the quartered onion, garlic cloves, bay leaf, lemon zest, salt and pepper. Top with water and bring to a boil. Lower the heat and allow to simmer for 30 minutes. Remove from the heat and allow to cool then strain and set aside until you are ready to cook the curry. (You can make the stock ahead and keep it in the fridge.)
To make the curry, heat the Weber Gourmet BBQ system wok over medium coals.
Add a splash of oil and fry the onion, garlic and ginger until soft and fragrant.
Add the spices and sugar and allow to fry for another 30 seconds before adding the prawn stock.
Allow this mixture to come to a simmer and cook for 20 minutes.
Add the cream, prawns and lemon then allow to simmer for 5 minutes until the prawns are cooked through but not rubbery then season to taste.
Serve topped with the fresh chillies (if using) and naan bread.