Over medium-high, heat oil in a large stock pot. Add the onion and cook until translucent {4-5 minutes}. Add the garlic and saute for an additional minute or until fragrant. Stir in masa flour and cook for an additional minute.
Pour in chicken stock slowly and stir until completely combined {you may need to use a whisk}. Then add chicken, enchilada sauce, black beans, Rotel, cumin and salt, stir until combined. Continue stirring until the mixture reaches a simmer, reduce heat to medium-low and let it simmer for 3-5 minutes {be sure to stir occasionally so it doesn't stick to the bottom of the pan}.
Serve soup warm with desired toppings and enjoy.