Drop the spinach into a saucepan of briskly boiling water and cook for 4 minutes. Drain, refresh in cold water, then drain thoroughly, pressing out as much water as possible. Rolling up the spinach in a bamboo mat and squeezing it is the easiest way, borrowed from the Japanese. Pur?e the spinach in a blender or food processor, and set aside. Dredge the salmon with flour, shaking to remove the excess. Season the fish with salt and pepper. Heat half the butter in a frying pan large enough to hold the fish in a single layer and saute the fillets, turning once, for about 6 minutes, or until done. Lift out of the pan onto a warmed plate, cover and keep warm. Add the rest of the butter and the leeks to the pan and cook, stirring, for 2-3 minutes over low heat until the leeks are tender. Add the fish stock and vermouth and simmer, uncovered, until the liquid is reduced to about half. Add the cream and reduce until the sauce is of coating consistency. Stir in the spinach and add salt and pepper if necessary. Spoon the sauce onto four heated plates and put a salmon fillet on top.