Preheat oven to 375°F (190°C). Lightly grease a baking dish and set it aside.
Bring a large pot of salted water to a boil. Cook spaghetti according to package directions until al dente. Drain and set aside.
In a large skillet over medium heat, melt butter or heat olive oil. Add garlic and cook for 30 seconds until fragrant.
Stir in the cooked chicken and spinach. Cook for 2–3 minutes, just until spinach is wilted.
Reduce the heat to low. Add heavy cream, salt, black pepper, and any additional seasoning. Stir gently until warmed through.
Remove skillet from heat and stir in grated Parmesan cheese until smooth.
Add the cooked spaghetti to the sauce and toss until evenly coated.
Transfer the mixture to the prepared baking dish. Sprinkle mozzarella (or cheese blend) evenly over the top.
Bake uncovered for 15–18 minutes, or until the cheese is melted and bubbly.
Remove from oven and let rest for 5 minutes before serving.