Begin by cooking the spaghetti according to package instructions until al dente. Drain and set aside.
In a large skillet, heat the olive oil over medium heat. Add the chopped onion and sauté until translucent, about 3-4 minutes.
Stir in the minced garlic, cumin, chili powder, and a pinch of salt and pepper. Cook for an additional 1-2 minutes until fragrant.
Add the shredded cooked chicken to the skillet, followed by the can of Rotel tomatoes and cream of chicken soup. Stir until well combined and the mixture is heated through.
Reduce the heat to low and gently fold in the cooked spaghetti and ½ cup of shredded cheddar cheese until everything is well mixed.
Transfer the mixture to a greased baking dish. Sprinkle the remaining cheddar cheese on top.
Bake in a preheated oven at 350°F (175°C) for 20-25 minutes, or until the cheese is melted and bubbly.
Remove from the oven and let it cool for a few minutes before serving. Garnish with fresh cilantro if desired.