Shredded Pork with Roasted Tomatoes and Chipotle Chiles

1. To make the tostaditas: Using a 2-inch round cookie cutter, cut 4 rounds from each tortilla. Pour 1/2 inch oil into a large, heavy skillet and heat over high heat until the oil is shimmering. Line a baking sheet with a double thickness of paper towels. Working in batches, add the tortillas to the oil and fry until golden brown, about 1 minute. Using a wire-mesh skimmer, transfer the tostaditas to the paper towels.
2. To braise the pork: Bring 8 cups water to boil in a large casserole or Dutch oven over high heat. Add the pork, onion, garlic, and salt and return to the boil. Reduce the heat to medium-low. Simmer, uncovered, until the pork is fork-tender, about 2 hours. Drain in a colander, discarding the onion and garlic, and let cool slightly. Transfer the pork to a chopping board and shred with 2 forks.
3. To make the sauce: Cook the chorizo in a medium nonstick skillet over low heat, stirring often and crumbling it with a wooden spoon until it turns dark red and some of the oil is released, about 15 minutes. Drain in a fine-meshed sieve to remove the excess oil, and transfer to a bowl.
4. Add the canola oil to the pan and heat over medium heat. Add the onion and cook, stirring often, until softened, about 3 minutes. Add the garlic and cook until fragrant, about 1 minute. Stir in the tomatoes, thyme, cloves, and cinnamon and bring to a simmer. Cover and reduce the heat to medium-low. Simmer until the tomatoes give off their juices, about 8 minutes. Add the chorizo, shredded pork, and chipotles and mix well. Cover and simmer to blend the flavors, about 2 minutes. Remove from the heat and add salt. (The pork in its sauce can be cooled, covered, and refrigerated up to 1 day ahead. Reheat slowly before serving.)
5. To serve, place 1 heaping tablespoon braised pork on each tostadita. Garnish with a drizzle of crema and a sprinkle of diced avocado. Serve hot.