Heat oil at the bottom of a pressure cooker. If you don't have a pressure cooker, you can use a normal pot, but adjust the cook time because it will take a little longer.
Slowly sear the cubes of pork in the heated oil, it doesn't have to be fully cooked at this point, it only needs to change color from pink to a light brown.
Add garlic, onion, water and salt to the pressure cooker. Cover and follow the instructions for the proper use of your pressure cooker. My pressure cooker takes about 15 minutes once the rocking valve at the top starts to rock.
Now peel, wash, and cut the tomatillos in half, and peel and cut the onion in quarters.
In a medium size pan, combine the tomatillos, onion, garlic, and enough water to cover. Cook over high heat. When the water boils, lower the temperature and let simmer for 5-6 minutes.
Meanwhile, remove the seeds and stems from the black chiles.
When the tomatillos are cooked, turn off the heat. Then, using a spoon, submerge the chiles in the hot water with the tomatillos, cover, and let rest for 15 minutes. Check from time to time to be sure the chiles stay submerged.
Pour the tomatillos, garlic, onion, cooked black chiles and all but 1/2 cup of the cooking liquid in a blender. Add thyme, salt, and vinegar and blend until completely incorporated.
In a large pan, heat the oil and pour in the salsa from the blender. Add in the pork from the pressure cooker and cook together for an additional 5-8 minutes.