1) In large pot of boiling water, cook pasta according to package directions; drain.
2) In small bowl, whisk together milk and the sauce packet; set aside.
3) In a large sauce pan or Dutch oven, melt butter over medium heat.
4) Add shallot and cook until translucent, about 2 minutes.
5) Add mushrooms and cook, stirring occasionally, until lightly browned, 5 to 6 minutes.
6) Pour in milk mixture and reduce heat to low.
7) Cook, stirring occasionally, until sauce is thickened, 3 to 5 minutes.
8) Add pasta, salmon and lemon juice, stirring and tossing until pasta is well coated and heated through, about 1 minute.