Preheat the oven to 190C/375F/Gas 5.
Fold two sheets of tin foil into large squares and butter the shiny sides.
Place a fillet on each buttered square of foil, flesh side down.
For each portion, top with 2 slices of lemon, 2 basil leaves, 25g/1oz of butter and some salt and pepper.
Fold the foil over the fish to make an envelope, and pour 50ml/2fl oz of vermouth into each pocket.
Leave for a few seconds to settle, then crimp the envelope edges to seal and place in the oven for 15 minutes.
Once cooked, remove the fish from the oven and strain the juice from each parcel into a saucepan.
Bring to the boil and reduce by half, then add the remaining butter.
Bring a saucepan of water to the boil, place a steamer on top then add the spinach and cover with a lid.
Steam until just wilted, then remove and season with salt and black pepper.
Bring a pan of salted water to the boil, then add the peas, boil for 3-5 minutes until just tender then drain and season with salt and black pepper.
To serve, put the fish with the lemon slices and basil onto warmed plates, then pour over the sauce. Serve with the spinach and peas.