Prepare the Cream Sauce
Preheat your oven to 375°F (190°C). Grease a 9x13-inch baking dish.
In a large skillet, melt butter over medium heat. Stir in flour and whisk constantly to make a roux.
Gradually add chicken broth and heavy cream, whisking until smooth.
Stir in garlic powder, onion powder, smoked paprika, salt, and pepper. Cook until the sauce thickens, about 5 minutes.
Combine Ingredients
In a large mixing bowl, combine the cooked spaghetti, shredded chicken, diced tomatoes and green chilies, and half of the shredded Monterey Jack cheese. Pour the sauce over the mixture and toss to coat evenly.
Assemble the Casserole
Transfer the spaghetti mixture to the prepared baking dish. Spread it out evenly.
Sprinkle the remaining Monterey Jack cheese (and cheddar, if using) over the top.
Bake the Casserole
Bake in the preheated oven for 20-25 minutes, or until the cheese is melted and bubbly.
Garnish and Serve
Remove from the oven and let cool for a few minutes. Garnish with fresh parsley before serving.