Preheat oven to 375°.
Spray a large baking dish with non-stick cooking spray and set aside.
Spray a large 13x9 baking dish with non-stick cooking spray.
Break the spaghetti in half and cook per the package instructions al dente.
Before draining the cooked pasta, save 2-1/2 cups and an extra ¼ cup of the pasta water.
Dissolve the bouillon cube into 2-1/2 cups of pasta water and set aside.
While the pasta is cooking, slice the mushrooms and chop the carrots, broccoli, and celery into bite-sized pieces.
Melt butter in a large skillet.
Add mushrooms and cook on medium heat for 3-4 minutes until they are soft.
Stir in flour. Mushrooms will become coated with the flour-butter mixture.
Slowly add the pasta bouillon water into the skillet, stirring constantly.
Add the broccoli, carrots, and celery.
Bring the mixture to a boil and reduce it to a simmer and cook for 2-3 minutes until it thickens.
Add paprika, nutmeg, salt, and pepper.
Reduce the heat and add the milk, extra 1/4 cup of pasta water, and 2/3 cups of cheese, and stir until the cheese is fully melted.
Add chicken and continue to stir.
Fold in the cooked pasta.
Transfer the mixture to a large baking dish.
Top with the remaining 1/3 cup of cheese and bake uncovered for 25 minutes.