In a large pot or Dutch oven, heat the olive oil over medium heat.
Add the onions and cook over medium-low heat until soft and translucent, about 15 minutes.
Add the garlic and cook for 1 more minute.
Stir in the chicken stock, tomatoes, salt, oregano, epazote, chile de arbol and pepper.
Bring the soup to a boil, then lower the heat and simmer for 15 minutes.
Using an immersion blender, blend everything until smooth. Check for salt at this time.
Add the alphabet pasta and bring the soup to a simmer and cook for 15-20 more minutes, stirring frequently, until the pasta is cooked through.