Cook the chopped onion and minced garlic in hot oil in a Dutch oven over medium-high heat, stirring constantly, until tender.
Add the refried beans and the next 3 ingredients, stirring until smooth; bring it to a boil.
Reduce the heat, and simmer it for 15 minutes.
Stir in cilantro, if desired.
Cut the corn tortillas into thin strips, spread them in a single layer on a baking sheet, and bake at 350°F for 15 minutes or until browned, stirring every 5 minutes.
Let them cool.
To serve, ladle the soup into bowls; top it evenly with tortilla strips, cheese, and sour cream.
