Preheat oven to 400 degrees F. Place tomatillos, tomato, jalapeno, and garlic cloves on a baking sheet lined with parchment paper or a silicone mat.
Group vegetables together and cover loosely with foil. Roast for 20 minutes.
Remove the foil and roast for another 10 to 15 minutes, until the vegetables begin to soften and brown.
Remove from the oven. Transfer vegetables to a food processor (or blender), and puree until the veggies are broken down and well combined.
Pork Roast:
Pour the Roasted Tomatillo Sauce into the crock pot. Add the green chiles, enchilada sauce, chicken broth, oregano, salt, cumin, and pepper; stir to combine.
Add the pork roast to the crock pot. Spoon the sauce over the pork. Add the onion slices, spreading them around the crock pot.
Cover and cook on HIGH for 3 hours, or until the pork is cooked through.
Carefully remove the pork roast to a cutting board. Cut the pork into 1/2-inch slices and then into bite-sized pieces.
In a small bowl, whisk together the cornstarch and water. Whisk the cornstarch mixture into the sauce in the crock pot.
Return the pork to the crock pot and cook on low for 20 to 25 mins.
Serve with warm tortillas, beans, and rice.
Notes
I would 100% recommend using a 3 pound pork shoulder/butt (yes, it's the same cut) instead of the pork loin. Either will work, but the shoulder will fall apart a little more and has more fat = magic in the mix. You will want to cook the shoulder for 5 to 6 hours on HIGH.