1. Put the pork roast in a large pot and cover with water. Bring the water to a boil and then turn down to a simmer. Simmer the meat for 3 to 4 hours, until very tender.
2. Shred the meat and grind in a meat grinder. Combine all the spices and sprinkle each batch of ground pork with spices. Mix well. Save the broth the pork was cooked in.
Soak the onion in the lime juice for 15 minutes in a small bowl. "Roast" the tomatoes, chili halves, and garlic clove in an ungreased skillet over high heat, turning as needed until slightly charred on all sides. Pulse in a blender; the mixture should remain chunky. Transfer to a bowl and add the onion, lime juice, and cilantro. Salt to taste. Use immediately, or cover and refrigerate for up to 1 week.