Preheat the oven to 350°F (175°C).

Boil the whole chicken in a large pot until fully cooked.

Remove the chicken from the pot and allow it to cool.

Once cooled, remove the chicken meat from the bones and shred it into bite-sized pieces.

In the same pot of water used to boil the chicken, cook the spaghetti according to package instructions.

Drain the spaghetti, but do not rinse it.

In a skillet, sauté the diced onion until it becomes translucent.

In a large mixing bowl, combine the Rotel tomatoes with chilies, cream of mushroom soup, diced Velveeta cheese, sautéed onion, and cooked spaghetti.

Mix well.

Transfer the mixture into a greased casserole dish.

Bake in the preheated oven for 30-40 minutes, or until the Velveeta cheese is melted and the casserole is heated through.

Remove the casserole from the oven and garnish with shredded Cheddar cheese.

Serve hot.