Cook pasta in salted boiling water, following directive on package. Drain and set aside in an oven dish.
Drain salmon, but keep the liquid. Mash the salmon with the bones. Set aside.
Melt 1/4 cup butter in a saucepan and add the flour. Stir 1 minute. Add milk and bring to boil, while stirring. Reduce heat and keep cooking stirring constantly, until thick. Set aside in a warm place.
Melt remaining butter in a skillet. Cook the onion 5 minutes, until tender. Add to the previous mixture of milk. Then add the salmon and his liquid. Add mozzarella and parsley. Add salt and pepper. Let heat while stirring, until cheese has melted.