Boil chicken until tender, cool, debone, and cut into pieces. Set aside. Reserve broth.
Cook spaghetti in broth until tender; drain. (NOTE: If you're using leftover chicken or turkey you can cook the spaghetti in purchased chicken broth. The broth adds a lot of flavor, but it's optional. If you don't have it, just cook the spaghetti in salted water.).
Place drained spaghetti in a very large bowl. Add cooked chicken.
In a skillet saute the pepper in butter. Add mushrooms and saute a few minutes more. Add to spaghetti along with remaining ingredients except the parmesan. Mix it well.
Put in a greased 9 x 13" casserole dish, and what's left in a second smaller greased casserole (this makes a lot. Don't worry, it freezes well and the leftovers taste even better the next day).
At this point you can cook both casseroles, or cover one and place in the freezer to enjoy later.
When you're ready to cook the casserole, sprinkle the top with parmesan cheese and bake at 350 degrees for 30-35 minutes, or until the top is lightly browned and bubbly.
REMEMBER: Don't add parmesan until baking time if you're freezing casseroles.