Preheat the oven to 350°F (175°C).

In a large pot, cook the spaghetti noodles according to package instructions.

Drain and set aside.

In a skillet, sauté the diced onion and bell pepper in butter until they are softened and translucent.

Add the cubed Velveeta cheese to the skillet and let it melt, stirring occasionally.

Once the cheese has melted, add the can of Rotel tomatoes to the skillet.

Stir until well combined.

In a large mixing bowl, combine the cooked and shredded chicken with the cooked spaghetti noodles.

Pour the cheese and Rotel mixture over the chicken and spaghetti.

Mix until everything is evenly coated.

Transfer the mixture to a greased 9 x 13 inch baking dish.

Bake in the preheated oven for 25-30 minutes, or until the edges are bubbly and golden brown.

Serve hot.