Bring a pot of water to boil for the pasta.
For the sauce, melt
butter in a saucepan
which will be large enough to accommodate the pound of cooked pasta, and saute onion until it becomes translucent. Mix in the flour to make a roux
. Gradually add milk
and allow thickening. Stir in tomato sauce and heat through. Remove from heat and add cream cheese
. Blend with an immersion blender
. Immediately cover surface of sauce with a sheet of plastic wrap
and set aside briefly.
Boil the pasta until al dente
. Add the cooked pasta and strips of salmon to the pan containing the sauce and fold to coat pasta. Season with salt and pepper, as needed. Transfer to a serving dish and sprinkle with chives.