Heat 3 T. butter in a small saute pan. Saute shallots until translucent. Add mushrooms. Turn heat to high, add salt and pepper, and cook until tender.
Lower heat; add salmon and stir rapidly. Add half the cream and continue cooking until thickened and reduced by half.
Meanwhile, bring a large pot of water to a rapid boil, salt water, and add linguini. Cook until al dente, about 2-3 minutes; drain.
Place a serving casserole over low heat. Melt remaining 3 T. butter, then add rest of cream. Combine. Add cooked linguini, and stir constantly while mixing in about half of the salmon and mushroom sauce. Add grated cheese and stir well.
Remove from heat and pour remaining sauce over linguini. Serve immediately.