Preheat the oven to 180°C.
Heat butter in a frying pan and soften spring onions, garlic, and mushrooms for 8-10 minutes.
Add white wine, bring to a boil, and simmer until liquid has almost evaporated.
Pour in cream and simmer until reduced by one third.
Season to taste and stir in lemon juice and chopped tarragon.
Dredge seasoned halibut fillets in flour and shake off excess.
Fry fillets in butter in an ovenproof pan for 2-3 minutes on each side.
Transfer the pan to the oven and cook for 6-8 minutes until opaque.
Place raw spinach on plates, top with halibut, and spoon over the sauce.