Melt half the butter in a frying pan. Add the fish and cook over a medium heat for 1-2 minutes on each side until lightly browned. Meanwhile melt the remaining butter in another frying pan and add the spring onions and cook over a low heat, stirring occasionally for 2 minutes until soft. Stir in the mushrooms and cover and cook, stirring occasionally for 3-4 minutes until lightly browned.
Using a fish slice, transfer the sea bass fillets to the mushroom mixture and pour in the cider. Cover and simmer gently for 2 minutes then season, with salt and freshly ground black pepper. Carefully transfer the fish to a warmed serving dish using a fish slice and keep warm. Stir the cream into the mushroom sauce. Blend the cornflour with 1 tbsp of water to form a smooth paste in a small bowl then stir the paste into the sauce. Increase the heat to medium and cook, stirring continually for 1-2 minutes until the sauce has thickened.
Serve the fish with the sauce spooned over garnished with parsley and served with roasted vine tomatoes.