Cook the spaghetti in a large pot of boiling salted water until al dente, following package instructions. Drain and set aside.
Heat olive oil in a skillet over medium heat. Add chopped onion and minced garlic; sauté until translucent and fragrant, about 3-4 minutes. Watch for the onion to become tender and slightly golden.
12 ounces spaghetti
Add the cooked diced chicken to the skillet with the sautéed vegetables, and cook for another 2-3 minutes until warmed through and lightly browned. Look for the chicken to develop a slight color and aroma of cooked meat.
12 ounces spaghetti
Pour in the can of Rotel diced tomatoes with green chilies, stirring to combine. Let the mixture simmer for 5 minutes, allowing flavors to meld and the liquid to slightly reduce, creating a fragrant, slightly thickened sauce.
12 ounces spaghetti
In a large mixing bowl, combine the cooked pasta, sautéed chicken and vegetables mixture, sour cream, 1 cup of shredded cheddar, and cumin. Mix thoroughly until everything is evenly coated and well combined.
12 ounces spaghetti
Transfer the mixture into a greased baking dish. Spread the remaining 1 cup of shredded cheddar cheese evenly over the top.
12 ounces spaghetti
Bake in a preheated oven at 375°F (190°C) for 20-25 minutes, or until the cheese is melted, bubbling, and slightly golden around the edges. The final dish should be hot and bubbly with a cheesy crust on top.