• Remove stems, split the peppers and discard the seeds. (Or, if you like your sauce pretty spicy, leave the seeds in.)
• Put chilies in a large pot, cover with water (at least 4 cups). Bring to a boil, remove from heat, cover and let stand 1 hour. Reserve 3 cups of soaking liquid. Place ½ of the peppers in a blender with 1 cup soaking liquid, and puree. Work the puree through a strainer into a bowl. Discard the bits of skin and any remaining seeds. Repeat with the remaining peppers. Use the final cup of liquid to rinse the blender and add to puree in bowl. You should have about 3½–4 cups of puree.
• Cook bacon in a large saucepan (preferably a Dutch oven) with the lard. When bacon is cooked but not yet crisp, add the onion. Cook until onion is translucent, stirring frequently so it doesn’t burn. Add flour, stirring constantly for about a minute. Mixture will be very dry and will burn easily, so don’t walk away from it.
• Add the chile puree, garlic, cumin, coriander, cinnamon, tomato, broth. Simmer over low heat for about 1 hour, stirring frequently to prevent scorching. Run sauce through strainer to remove bacon and onion.
• Stir in vinegar, oregano and salt. If sauce is too spicy for your taste, add more tomato.