Bring a large pot of salted water to a boil. Cook the spaghetti until al dente according to package directions. Drain and set aside, reserving about ½ cup of pasta water.

In a bowl, toss the diced chicken with salt, black pepper, paprika, garlic powder, and Italian seasoning until evenly coated.

Heat olive oil in a large skillet over medium-high heat. Cook the chicken for 6-8 minutes until lightly browned and cooked through. Transfer to a plate.

In the same skillet, melt the butter. Add the diced onion and bell pepper and cook for about 5 minutes until softened. Stir in the minced garlic and cook 30 seconds more.

Sprinkle the flour over the vegetables and cook for about 1 minute, stirring constantly, to remove the raw flour taste.

Gradually whisk in the chicken broth, milk, and heavy cream. Cook until the sauce begins to thicken, then stir in the cream cheese, Monterey Jack, cheddar, smoked paprika, and black pepper until smooth.

Add the cooked chicken back into the skillet along with the cooked spaghetti. Add a splash of reserved pasta water if needed to loosen the sauce, and toss until fully coated.

Transfer the mixture to a lightly greased 9x13-inch baking dish and sprinkle the remaining Monterey Jack cheese evenly over the top.

Bake at 375°F (190°C) for 15-20 minutes until the cheese is melted and bubbly. Broil for 2-3 minutes for a golden top, watching closely.

Let the casserole rest for 5-10 minutes, then sprinkle with fresh parsley before serving.