Bring a large pot of salted water to a boil and cook the pasta according to package directions. Drain and return the hot pasta to the empty pot. Cover with lid.
Meanwhile, shred the cheese and chicken. And microwave the broccoli according to package directions. Chop the broccoli as needed into small, bite-sized pieces.
In a large bowl or glass measuring cup, combine the melted butter with the cream of chicken soup, mayonnaise, sour cream, half the shredded cheese, lemon juice, parsley and salt. Whisk or stir well to combine.
To the pot with cooked pasta, add the prepared chicken and broccoli and the sauce. Mix very well to combine.
Evenly spread the pasta mixture into the casserole dish. Top with the remaining shredded cheddar cheese. Then top with the fried onions.
Bake the casserole uncovered for about 30 minutes, until hot and bubbly. Serve immediately.