Boil the Pasta
Start by bringing a large pot of salted water to a boil. Cook the spaghetti according to package directions until it's just al dente. It’s important not to overcook the pasta at this stage, because it’ll continue cooking in the oven later. Once done, drain the pasta and set it aside.
Make the Creamy Tomato Sauce
While the pasta boils, heat olive oil in a large skillet over medium heat. Add diced onions and cook until they’re soft and translucent, which usually takes about 3–4 minutes. Stir in the garlic and sauté for another 30 seconds until fragrant—don’t let it burn. Now pour in the marinara sauce and stir in the heavy cream. This is where the magic happens! Let the sauce simmer for 4–5 minutes so it thickens slightly and the flavors come together.
Season and Add Chicken
Season the sauce with Italian seasoning, salt, and black pepper. Then stir in the cooked, shredded chicken. If the chicken was cold, let it warm up in the sauce for a few minutes. This allows the flavors to blend and keeps the chicken juicy and tender. Make sure the sauce is evenly mixed and well-seasoned—taste and adjust if needed.
Combine Pasta and Sauce
Add the drained spaghetti directly into the sauce mixture. Toss it all together gently until the noodles are evenly coated in that creamy, cheesy, tomato-rich sauce. At this point, everything should look and smell irresistible. It’s okay if it feels like there’s a lot of sauce—it’ll absorb more while baking.
Assemble and Bake
Preheat your oven to 375°F (190°C). Transfer the spaghetti mixture into a greased 9x13 baking dish. Spread it out evenly. Sprinkle the top with shredded mozzarella and parmesan cheese, covering the surface generously. Bake for about 20–25 minutes, or until the cheese is melted and bubbly, and the edges are lightly golden.
Let It Rest and Serve
Once baked, let the dish rest for 5–10 minutes before serving. This helps it set slightly and makes it easier to serve. I like to garnish with a little chopped parsley for color and freshness, but that’s totally optional. Serve hot and watch it disappear quickly—it’s that good.