Prepare the spinach by transferring it with all the water that clings to the leaves after washing to a saucepan. Cook over medium-low heat, stirring occasionally, until tender but still with some body. Remove the spinach to a bowl and set aside.
In the same saucepan, make a bechamel sauce over medium-low heat by stirring the 2 Tbs. butter and the flour together until the two are blended. Then whisk in the milk, a little at a time, until it blends into a fluffy sauce. Add the cheeses and stir in until incorporated. Add the spinach back to the saucepan and combine with the sauce. Set aside.
Blend the salt, pepper, and lemon juice into the olive oil. Dip the salmon fillets in the oil, shake off the excess, and place on a hot grill or under a preheated broiler on a preheated rack. Sear the salmon for about three minutes on each side, then set aside and keep warm. (Test for doneness: insert a kitchen fork into the thickest part of the fish, hold it there for five seconds, then carefully touch the tines to your lips. If they feel warm at all, the fish is ready.)
While the salmon is cooking, make the sauce. Combine the shallots, peppercorns, bay leaf and wine together and bring to a boil. Cook until most of the wine has evaporated. Add the cream and reduce the liquid by about one-third.
Strain the sauce into a warm skillet. Whisk in the butter, a little at a time, over very low heat. Add the lemon juice and the capers.
Divide the spinach among four plates. Place the salmon fillets atop the spinach, and nap with the sauce.
