Instructions

• Preheat oven to 350°F and move the oven rack to the top third. Soften the cream cheese by microwaving it 20–30 seconds or leaving it at room temperature. Chop the 1/2 medium onion, mince the garlic, slice the mushrooms, and cut the 2 chicken breasts into 1" pieces; season the chicken with salt and pepper to taste.
• Bring a large pot of salted water to a boil for the spaghetti.
• Cook the 12 ounces uncooked spaghetti until about 2 minutes less than the package time (slightly under al dente). Drain the pasta and set it aside.
• While the pasta cooks (or immediately after), heat 1 tablespoon olive oil and 1 tablespoon of the butter in a large pot or deep skillet over medium-high heat.
• Add the chopped onion and sauté 4–5 minutes, until the onion is lightly browned.
• Add the seasoned chicken pieces to the pot and cook 5–6 minutes, until the chicken is almost cooked through. Transfer the chicken to a plate, leaving any onions in the pot.
• Add the 4 cloves minced garlic, the remaining 2 tablespoons butter, and the 1/4 teaspoon Italian seasoning, then add the 7 ounces sliced mushrooms. Cook about 5 minutes, stirring occasionally, until the mushrooms release their liquid and most of it has evaporated.
• Increase the heat to high and add 3/4 cup chicken broth, 1 cup heavy cream, and the softened 8 ounces cream cheese (cut into smaller pieces). Stir nearly constantly for about 3–5 minutes until the cream cheese melts and the sauce becomes smooth and slightly thickened. Avoid cooking longer than needed to prevent the sauce from becoming too thick.
• Turn off the heat. Return the cooked chicken to the pot and add the drained spaghetti. Stir well so the pasta and chicken are evenly coated with the sauce.
• Transfer the mixture to a 9×13-inch baking dish and spread it into an even layer. Sprinkle 1 cup freshly grated Parmesan evenly over the top, then sprinkle 1 cup shredded mozzarella over the Parmesan.
• Bake uncovered at 350°F for 30 minutes. After 30 minutes, broil for a few minutes until the top is browned to your liking—watch carefully to avoid burning.
• Remove from the oven and let the dish rest about 5 minutes before serving. Use a spatula to cut and serve so each portion has melted cheese on top.