Remove legs from shrimp and discard. Remove middle part of shells, leaving
heads and tails. Reserve shells. Devien shrimp.
Place shrimp shells, quartered onion, 2 garlic cloves, bay leaf, and 4 cups of
water in a large stock pot. Bring to a boil, then simmer for 20 minutes.
Pour through a fine sieve, pressing the solids to remove as much liquid as
possible. Reserve the broth and discard the solids.
Combine tortillas, tomatoes, remaining garlic, chilies or pepper pieces in
blender and process until slightly smooth - about 40 seconds.
Add 2 tbsp oil to stock pot. Heat until very hot. Pout tomato mixture through
a sieve into stock pot, pressing solids. Discard remaining solids. Cook,
stirring often, about 10 minutes, until thickened.
Whisk in shrimp stock, bring to a boil, and reduce heat to simmer, whisking
occasionally, for 20 minutes. Season with salt and pepper.