Season the steaks with celery salt and drizzle with extra virgin olive oil. Arrange on a hot barbecue grill and cook for 10 to 12 minutes, turning regularly, until medium-rare. Set aside to rest for 3 minutes.
Put half the butter and the flour in a medium saucepan with the Herbes de Provence and cook until thickened. Whisk in the wine and bring to a simmer, then add the stock and simmer until thickened. Mix in the crème fraiche. Season with salt and pepper, then fold in the tarragon.
Arrange the prawns on a tray and top with the remaining butter. Grill until lightly browned. Place the steaks on trays and slather with sauce. Top with prawns. Serve with kale and parsley.
COOK’S NOTES: Local Manning River prawns are available from September to January. Smaller in size and extremely sweet, they would complete the local flavours of this dish.