In a medium sized skillet, add olive oil and heat over medium high heat. Brown the sides of the pork and add them to the slow cooker.
In a food processor add the tomatillos and blend until smooth. Add it to the slow cooker alone with chopped onion, garlic, green enchilada sauce, salsa verde, green chilies, cumin, dried oregano, and salt.
Cook on low for 6 hours or on high for 4. An hour before serving, take out 1 cup of juice from slow cooker and whisk it with the cornstarch. Add it back to the slow cooker and allow to thicken and cook for about 1 more hour.