First, gather and prepare all your ingredients. Cut the chicken breasts into even pieces, chop your onions and bell peppers, mince the garlic, and slice the mushrooms. Set everything aside within easy reach.
Start by cooking your pasta. Fill a large pot with water, add salt, and bring to a boil. Add the elbow macaroni and cook for about 7 minutes – slightly less than what the package says since it will cook more in the oven. You want the pasta still a bit firm. Drain and set aside.
For the chicken, place the pieces in a deep pot and add enough water to cover them. Add salt and peppercorns. Bring to a boil, then lower the heat right away to a gentle simmer. Cook for 10-15 minutes until the chicken reaches 165°F. Take the chicken out and let it cool a bit before shredding it into bite-sized pieces.
Now make your sauce. Heat canola oil in a large pot over medium heat. Cook the onions until they turn clear, about 2-3 minutes. Add garlic and cook until you can smell its aroma, about 30 seconds. Put in your mushrooms and bell peppers, cooking until they soften, about 3-5 minutes.
Pour in the sweet-style spaghetti sauce and a cup of water. Add your chicken bouillon cube and Italian seasoning. Stir well and let it gently simmer. Cook for 5 minutes, then add your shredded chicken. Taste and add salt and pepper as needed.
Turn your oven on to 375°F. While it's heating up, combine your cooked macaroni with the chicken and sauce mixture in a large bowl. Mix everything well so the pasta is evenly coated. Pour it all into a greased 9x13-inch baking dish.
Cover the top generously with shredded quick-melt cheese, making sure to spread it evenly. Cover the dish with foil, but be careful not to let it touch the cheese.
Put it in the oven and bake for 20 minutes. Then take the foil off and bake for another 5-7 minutes until the cheese turns golden brown and starts bubbling. Take it out of the oven and let it rest for about 10-15 minutes before serving – this helps the sauce set and makes it easier to serve.
When you're ready to serve, cut into squares while it's still warm and the cheese is stretchy. It goes perfectly with garlic bread and your favorite cold drink. If you have leftovers, keep them in a covered container in the fridge for up to 3 days. When reheating, add a few drops of milk or water to keep it moist, and warm it slowly for the best results.