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 a 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.
Preheat oven to 375 F.
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 the 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.