Place Whole Chicken (1) in a large stockpot with the Onion (1) and Celery (2 stalk).
Fill with cold water and cook chicken until falling off the bone. The time will depend on the size of your chicken. Save the chicken broth when it is cooked through, you'll use it in the casserole.
Let the chicken cool, then de-bone and chop up. Measure out 3 cups of meat and set aside. Any extra chicken can be used in soups, sandwiches, salads or any other recipe that needs cooked chicken.
Meanwhile, strain the chicken broth into a clean stock pot. Reserve some of the broth for the sauce.
Bring your remaining chicken broth to a boil in the fresh stock pot. Break Spaghetti (16 ounce) in half and add it to the pot.
Boil spaghetti until slightly underdone (a minute or two under package directions). Drain and rinse with cold water to stop the spaghetti from cooking any more. Set aside.
Preheat the oven to 350 degrees F (180 degrees C).
In an extra-large skillet or stockpot, melt Butter (1/2 cup) and add Onion (1 cup), Mushroom (1 cup), Green Bell Pepper (1/2 cup), and Pimiento Pepper (1 jar). Stir and sauté until all vegetables are tender.
Add Whole Wheat Flour (2 tablespoon) as you mix to make a roux.
Add Cream (1 1/2 cup), stirring the mixture as you pour. Next add 1 cup of the reserved chicken broth.
Add Diced Tomatoes (2 can). Mix gently, add Salt (to taste), Ground Black Pepper (to taste) and Cayenne Pepper (1/8 teaspoon).
Add the cooked spaghetti pasta and mix it all together well.
Stir in Cheddar Cheese (2 1/2 cup)
Add cooked chicken. Stir together well and put into one large lasagna pan, or 2 medium/small casserole pans.
Top with remaining Cheddar Cheese (1 1/2 cup).
Bake in preheated oven 30-40 minutes, or until bubbling and heated through. I usually cover with foil for 20 minutes, then remove the foil and finish cooking so the cheese can get extra bubbly.
Let cool for a few minutes after you take it out of the oven, then serve warm in large bowls.