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Remove the stems and seeds from the red chiles. Cut into 1" strips.
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Add olive oil to a large pan over medium heat. Add in chiles, onion, and garlic. Saute for 5 - 10 minutes, until fragrant. Pour in water, salt, and oregano, stirring well. Bring to a boil, cover, then reduce heat to a simmer for 10 minutes.
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Transfer chile mix to a blender, in batches if necessary, and blend until smooth. Strain red chile sauce through a sieve to filter out peels.
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Stir red chile sauce into the pork. Cover and refrigerate for 12 - 24 hours.
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Cook in slow cooker on low for 9 hours, until cooked through and fork-tender.