In a large pot over medium heat, bring about 4 quarts salted water to a boil. Add macaroni and cook 1 to 2 minutes less than package's directions. Rinse under cold running water and drain well. Set aside.
In small, heavy-bottomed pot, arrange chicken breasts in a single layer. Add enough water to cover, salt and peppercorns.
Bring to a boil, skimming scum that floats on top. Lower heat, cover, and cook in a barely a simmer for about 10 to 15 minutes or until thermometer inserted in the thickest part of chicken reads 165 F.
With a slotted spoon, remove chicken from heat and allow to cool to touch. Coarsely shred chicken and keep warm.
In a large pot over medium heat, heat oil. Add onions and garlic, and cook until softened.
Add mushrooms and bell peppers and cook for about 3 to 5 minutes or until softened.
Add spaghetti sauce and water. Bring to a boil.
Add chicken bouillon, Italian seasonings, and salt and pepper to taste. Lower heat, cover, and simmer until slightly thickened.
Add chicken and continue to cook for about 3 to 5 minutes or until heated through.
In a large bowl, combine macaroni and sauce. Gently toss together until well-combined.
Lightly grease the bottom and sides of a large baking dish with nonstick cooking spray. Transfer pasta-meat mixture into prepared dish. Sprinkle shredded cheese evenly on top.
Bake in a 375 oven for about 20 to 25 minutes or until hot and cheese is melted.
Remove from oven and allow to cool for about 7 to 10 minutes before cutting into serving portions.