Cook the pasta: Bring a large pot of salted water to a boil. Add spaghetti and cook until al dente (about 1-2 minutes less than package directions). Drain well and set aside.

Sauté the aromatics: In a large, oven-safe skillet over medium heat, melt the butter. Add the chopped onion, bell pepper, and celery. Sauté for 5-7 minutes until softened. Add minced garlic and cook for 1 more minute until fragrant.

Build the sauce: Pour in the chicken broth to deglaze the pan. Stir in the cream of chicken soup, undrained Rotel, garlic powder, onion powder, smoked paprika, salt, and pepper until combined.

Melt the cheese: Reduce heat to low. Add the cubed Velveeta and 1 cup of the shredded cheddar. Stir constantly until the cheeses are fully melted and the sauce is smooth and velvety.

Combine everything: Turn off the heat. Add the cooked spaghetti and shredded chicken to the skillet. Use tongs to toss gently until everything is thoroughly coated in the cheese sauce. Add a splash more chicken broth if the mixture is too thick.

Broil and serve: Preheat your oven's broiler to high. Smooth the top of the spaghetti mixture and sprinkle with the remaining 1 cup of cheddar cheese. Place under the broiler for 2-4 minutes, watching closely, until the cheese is bubbly and golden-brown. Let rest for 5 minutes before serving.