Remove any fat from chicken and cut into 1” cubes.
Place olive oil, chicken pieces and onion in a large skillet over medium heat. Cook, stirring occasionally until the chicken pieces are cooked through. During last minute of cooking add minced garlic. Cook until fragrant, about a minute. Remove chicken from skillet and set aside.
Pour 1 c. of milk into skillet, over low heat. Slowly whisk in flour until smooth. Add the rest of the milk and chicken broth whisking as you add the liquid to keep it smooth.
Whisk in salt and pepper, bring to a boil and allow to thicken over medium heat, whisking frequently to keep the sauce smooth.
Stir in Rotel and uncooked spaghetti noodles, turn up heat to high and bring the mixture back to a boil. Reduce heat to medium-low and cover. Cook for 8-10 minutes, stirring occasionally or until pasta is cooked to al dente.
Cut cream cheese into bite size pieces. Stir into pasta along with Mexican cheese. Add chicken and heat through.
Garnish with tomato and serve.
