Preheat oven to 350 degrees. Grease a large casserole dish. Slice mushrooms, separate broccoli, dice onions and cube the raw chicken. Boil the pasta (start next step while it's cooking), drain and set aside.
Saute chicken and onions in butter with salt until chicken is white, stirring constantly. Sprinkle with lemon juice, add mushrooms and and cook an additional 3 minutes at reduced heat. Transfer chicken and onions to casserole dish, reserving any juices in the frying pan.
Add flour to the juices in the frying pan, cook and stir for 2 minutes. Add cream and salt, bring to a boil, whisking constantly. Reduce heat and simmer to thicken, still whisking. Add 75% of the Swiss cheese, continue to simmer and stir until cheese melts.
Pour cream sauce over chicken in the casserole dish. Add noodles and broccoli, gently stir to combine. Top with remaining Swiss cheese.
Bake covered at 350 degrees for 35 minutes or until bubbly throughout.