Step 1: Preheat your oven to 350°F (175°C). Grease a 9x13 inch baking dish.
Step 2: Cook the spaghetti according to package directions until al dente. Drain well and set aside.
Step 3: While the spaghetti cooks, heat olive oil in a large skillet or Dutch oven over medium heat. Add the diced onion and bell pepper and sauté until softened, about 5-7 minutes.
Step 4: Add the minced garlic and cook for another minute until fragrant. Be careful not to burn the garlic.
Step 5: Stir in both cans of cream of mushroom soup and cream of chicken soup, the undrained Rotel, chicken broth, milk, Italian seasoning, garlic powder, and black pepper. Bring the mixture to a gentle simmer, stirring occasionally.
Step 6: Remove the sauce from the heat. Stir in the shredded cooked chicken and 1.5 cups of the cheddar cheese and 0.5 cups of the Monterey Jack cheese until melted and fully incorporated. This forms the creamy base for your creamy chicken spaghetti recipe.
Step 7: Add the cooked spaghetti to the sauce mixture and toss gently until all strands are evenly coated.
Step 8: Pour the spaghetti mixture into the prepared baking dish, spreading it out evenly.
Step 9: Sprinkle the remaining 0.5 cup of cheddar cheese and 0.5 cup of Monterey Jack cheese over the top of the creamy chicken spaghetti.
Step 10: Bake for 25-30 minutes, or until the cheese is bubbly and lightly golden and the creamy chicken spaghetti recipe is heated through. Let stand for 5-10 minutes before serving. Garnish with fresh parsley if desired.