1. Preheat your oven to 375°F (190°C). Lightly grease a 9×13-inch baking dish to prevent sticking.
2. Bring a large pot of salted water to a boil. Cook the spaghetti according to package instructions until al dente, usually about 8-9 minutes. Drain and set aside.
3. While the pasta cooks, heat olive oil in a large skillet over medium heat. Add the chopped onion and sauté until translucent, about 4 minutes.
4. Add the minced garlic and cook for another minute until fragrant, being careful not to brown the garlic.
5. In a separate saucepan, melt the butter over medium heat. Whisk in the flour and cook for 1-2 minutes, stirring constantly to create a roux.
6. Slowly whisk in the milk or half-and-half, continuing to stir until the sauce thickens and coats the back of a spoon, about 5 minutes. Season with salt, pepper, and Italian seasoning.
7. Combine the cooked pasta, sautéed onions and garlic, shredded chicken, marinara sauce, and the creamy milk sauce in a large bowl. Stir until everything is evenly mixed.
8. Transfer the mixture into your prepared baking dish. Sprinkle mozzarella, cheddar, and Parmesan cheeses evenly on top.
9. Bake uncovered for 25-30 minutes, or until the cheese is melted, bubbly, and golden around the edges.
10. Remove from the oven and let it rest for 5 minutes before serving. Garnish with fresh basil or parsley if you like.

One little trick I’ve learned from my travels in Italy is to let the bake rest before serving. It allows the flavors to marry and the cheese to set just right, making each bite more satisfying—a lesson I happily pass on to my daughters during our kitchen adventures.