Preheat the oven to 350℉.
Cook the linguini according to the package instructions in salted water. For easier serving and mixing, break the pasta into smaller pieces as you add it to the boiling water. Once cooked to al dente, drain the pasta and set it aside
In a saucepan over medium-high heat, melt the butter and add the flour. Whisk the mixture together until it becomes bubbly, forming a roux. Continue to cook the roux for another 2 minutes to reduce the flour taste in the final dish.Gradually whisk in the chicken broth and cook until the sauce thickens. Lower the heat and add the cream, mushrooms, salt, pepper, and cheeses. Stir often and continue to cook until the cheese is fully melted and the sauce is smooth. Add the cooked pasta, green onions, and mushrooms to the sauce, stirring to combine everything evenly. Pour the mixture into a casserole dish and sprinkle a little extra cheese on top.
Place in a preheated oven for 30 minutes, or until the casserole is bubbly and lightly browned on top.
