Preheat the oven to 180C/350F/Gas 4.
For the sauce, heat the butter in a frying pan over a medium heat. Add the spring onions, garlic and mushrooms and fry for 8-10 minutes, or until softened.
Add the wine and bring the mixture to the boil. Reduce the heat until the mixture is simmering, then continue to simmer until the liquid has almost completely evaporated.
Add the cream and continue to simmer until the volume of liquid has reduced by one third. Season, to taste, with salt and freshly ground black pepper, then stir in the lemon juice and tarragon.
Meanwhile, for the halibut, sprinkle the flour onto a plate and season, to taste, with salt and freshly ground black pepper. Dredge the halibut fillets in the flour until completely covered, shaking off any excess.
Heat the butter in an ovenproof frying pan over a medium heat. Add the coated halibut fillets, in batches if necessary, and fry for 2-3 minutes on each side.
Transfer the pan to the oven and continue to cook the halibut for a further 6-8 minutes, or until just cooked through. (NB: The halibut is cooked through when the flesh is opaque.)
To serve, place a handful of raw spinach leaves into the centre of each of four serving plates. Place one halibut fillet on top of each. Spoon over the spring onion cream sauce.