Preheat your oven to 350ºF (175ºC). This ensures the dish bakes evenly and thoroughly. While the oven is heating, prepare your baking dish by lightly spraying a 9×13-inch pan with cooking spray to prevent sticking.
Cook the spaghetti according to the package directions until al dente. Drain the pasta in a colander and set it aside. You want the spaghetti to retain some firmness, as it will continue to cook in the oven.
In a large mixing bowl, combine the chopped cooked chicken, cream of chicken soup, sour cream, the drained spinach, 1 cup of Monterey Jack cheese, and minced garlic. Mix everything together until well combined. The mixture should be creamy and evenly distributed.
Gently stir in the drained spaghetti until it's coated with the sauce. Ensure the pasta is fully enveloped in the creamy mixture, which will help it retain moisture while baking.
Pour the spaghetti mixture into your prepared baking dish, spreading it evenly. This ensures every bite has the perfect balance of chicken, sauce, and spaghetti.
Sprinkle the remaining shredded Monterey Jack cheese and half a can of the French Fried Onions on top. This will create a golden, crispy layer as it bakes.
Place the dish in the preheated oven. Bake uncovered for 40 to 50 minutes, or until the top is golden brown and the edges are bubbling. Keep an eye on it during the last few minutes to prevent over-browning.
Once baked, remove the dish from the oven and let it cool for a few minutes before serving. This allows the sauce to settle a bit, making it easier to serve without falling apart.
Lastly, serve your Monterey Chicken Spaghetti warm, garnished with the remaining French Fried Onions for extra crunch. Enjoy your delightful, comforting creation!