tortilla soup

1/2 white onion
1 pound plum tomatoes
6 peeled garlic cloves
2 [dried] guajillo chilies* (about 1 ounce)
2 [dried] ancho chilies* (about 1 ounce)
8 cups chicken stock*
1/2 teaspoon dried oregano, crumbled
1 teaspoon coarse salt

ten 5- to 6-inch white corn tortillas
about 2 cups peanut or vegetable oil for deep-frying


8 fresh coriander sprigs
Garnish: lime wedges