For the sea bass, melt the butter in a sauté pan over a medium heat. Add the chopped onion and sauté for 4-5 minutes, or until softened.
Add the mushrooms to the pan and cook for another 2-3 minutes. Season, to taste, with salt and freshly ground black pepper.
Pour in the stock, bring to the boil, then reduce the heat until the mixture is simmering. Place the sea bass into the mixture, cover the pan with a lid and poach for 6-8 minutes, or until the sea bass is cooked through and opaque. Remove the sea bass from the pan and set aside to rest on a serving plate.
Pour the cream into the poaching liquid, return to the boil and cook for 3-4 minutes, or until the volume of the liquid has reduced. Season, to taste, with salt and freshly ground black pepper, then stir in half of the chopped chives.
Meanwhile, for the leeks, melt the butter in a frying pan over a medium heat. Add the blanched leeks and fry for 2-3 minutes. Season, to taste, with salt and freshly ground black pepper.
To serve, pour the cream sauce over the fish, spoon the leeks alongside and sprinkle with the remaining chopped chives.
