Melt 20g of butter in saucepan. Add mushrooms and shallots then cook over low to medium heat until shallots are soft. Add wine and boil until reduced by half. Add cream and reduce by half or until thickened. Season with salt, pepper and lemon juice.
Smear all but 20g of the remaining butter on the salmon, Season with salt and pepper then cook under a pre-heated grill for 8 – 10 minutes until done (time will depend on thickness).
Meanwhile, melt the remaining butter in a frying pan. Add spinach and saute until wilted. Add peas to sauce and warm through. Divide spinach between plates, top with salmon and trail sauce over.