Cook the Chicken: Heat 1 tbsp olive oil in a large skillet over medium-high heat. Season chicken breasts with salt and pepper. Cook for 6-8 minutes per side, or until cooked through (internal temperature 165°F/74°C). Remove from skillet, let cool slightly, then shred or dice. Set aside.

Cook the Spaghetti: While chicken cooks, bring a large pot of heavily salted water to a rolling boil. Add spaghetti and cook according to package directions until al dente. Drain well and set aside.

Sauté Aromatics: In the same large skillet (or a clean one) over medium heat, add 2 tbsp olive oil. Add chopped onion and bell pepper and cook for 5-7 minutes, or until softened. Add minced garlic and cook for another minute until fragrant.

Simmer Sauce: Stir in the cream of mushroom soup, undrained diced tomatoes, chicken broth, and milk. Bring to a gentle simmer, then reduce heat to low. Add the cubed Velveeta cheese, Worcestershire sauce, chili powder, smoked paprika, dried oregano, salt, and pepper. Stir continuously until the Velveeta is completely melted and the sauce is smooth and creamy. Taste and adjust seasonings as needed.

Combine: In a very large bowl, combine the cooked and shredded chicken, drained spaghetti, and the creamy cheese sauce. Mix thoroughly until everything is well coated.

Prepare for Baking: Preheat your oven to 375°F (190°C). Lightly grease a 9x13 inch baking dish. Pour the spaghetti mixture into the prepared dish, spreading evenly.

Bake: Sprinkle 1.5 cups of the shredded cheddar cheese evenly over the top. Cover the baking dish tightly with aluminum foil and bake for 25 minutes. Remove the foil, sprinkle with the remaining 1/2 cup cheddar cheese, and bake uncovered for an additional 10 minutes, or until the cheese is melted and bubbly and the casserole is heated through.

Serve: Let the casserole rest for 5-10 minutes before serving. Garnish with fresh chopped parsley, if desired. Enjoy your delicious Creamy Baked Chicken Spaghetti!