Set a large skillet over medium heat and add the oil. Once hot, add the onion and tomatoes and cook, stirring frequently, for 5 minutes.
Add the tomatillo salsa and garlic, cook 1 more minute. Stir in the chipotle and adobo and remove from the heat.
Allow to cool slightly, then transfer to a blender along with the broth or water and carefully puree until smooth (alternatively, you can use an immersion blender to puree).
Place pork in the bottom of a 5- to 6-quart slow cooker.
Pour pureed sauce over the pork, then cover with a lid and cook on low for 8 to 9 hours.
Using two forks, shred the pork (this can be done inside the slow cooker).
Toss with the sauce and cook an additional 10 to 15 minutes to meld flavors. Season to taste with salt and pepper.
Serve pork in warmed tortillas, then top with shredded lettuce, shredded cabbage, thinly sliced radish, and cotija cheese.
RECIPE NOTE: I know it seems like a pain to cook something before slow cooking it, but I promise this is worth it!
SERVING TIP: Pile the veggies on for these tacos! The crunchier, the better!