Bring a large pot of water to a boil for the pasta.
Heat the butter in a large skillet; when it begins to bubble add the shallot. Saute the shallot until translucent but not browned. Add the cream, bring it to a boil then lower it to a simmer. Cook the cream until it reduces by half and thickens slightly. Season the cream with the salt and pepper.
Add the salmon to the simmering cream. Gently stir it for a couple of minutes until it is just poached. Drop the pasta into the pot of boiling water, and add the spinach to the cream and salmon. Cook the pasta for about three minutes--until al dente--and drain it thoroughly. Cook the spinach until it is just wilted. Add the lemon zest to the sauce; stir gently, then add the cooked and drained pasta to the sauce. (If the saute pan is not large enough to accommodate both the sauce and the pasta transfer both of them to a large warmed bowl.) Gently toss the pasta and sauce together.