Bring the pork, quartered onions, smashed garlic, thyme sprigs and 6 cups water to a boil in a large saucepan over medium-high heat. Skim off any fat foam that rises to the surface. Reduce heat to medium-low and simmer for 90 minutes. Drain pork, reserving 1 cup of cooking liquid. Discard the onion, garlic and thyme. Let pork cool on a baking sheet. Once cool enough to handle, finely chop pork into 1/4 inch pieces.

Heat oil in a nonstick skillet over medium-high heat until shimmering. Add chopped pork, oregano and salt and cook, stirring often, until pork is well browned and crisp.

Stir in tomato sauce and chipotle powder and mix well. Add reserved cooking liquid and simmer until liquid has evaporated, about 5 minutes.

Spoon small amount of chopped pork onto center of each tostada and serve. Add cheese, cilantro and sour cream separately.