Bring about 3 cups of water to a boil in a covered
pot and add the shells and stir. While the shells are cooking, prepare the
other ingredients.
Melt the butter in a large pan over low heat and
slowly stir in the flour making a paste.
Combine the milk and chicken broth and slowly stir
it in, a little at a time, dissolving the paste. Add salt and pepper to
taste.
Add chicken and mushrooms and raise heat to low
boiling point to thicken the sauce. When the sauce reaches the desired
consistency, turn off the heat. Don't let the sauce get too thick, as it
will thicken more when the pasta is added.
Turn on the broiler in the oven.
When the pasta is al dente, drain it and stir it in.
Sprinkle the cheese over the top and place the pan
under the broiler.
When the cheese is melted and golden, remove and
serve.