Prep Meat- In a small bowl, mix together all spice-rub ingredients and rub all over Boston Butt. Place meat in slow cooker with the fat cap facing up.
Prep Sauce & Slow Cook- Add all sauce ingredients to a blender and blend until smooth. Pour mixture in to slow cooker over meat. Add in minced garlic and 1 cup chopped onion. Pour water over everything. Set slow-cooker to low for at least 10 hours.
Fry Meat- Once slow-cooker is done, remove meat with tongs and allow to rest for 10 minutes. Save all slow-cooker sauce, you will need it to fry meat and tortillas (plus some for dipping). Remove bone and use 2 forks to shred meat. Working in batches, fry shredded meat by placing a single layer of meat in a skillet over high heat. Pour 1/2 cup of sauce from the slow cooker over meat. Allow to fry undisturbed for 4 minutes until the bottoms of the meat are crispy. Once crispy, stir meat and cook 1-2 minutes longer. Set shredded crispy meat aside.
Fry Tortillas & Construct- Dip a corn tortilla in slow-cooker sauce. Fry tortilla in same skillet for 30 seconds on each side over high heat. Sprinkle some shredded cheese, crispy meat, and chopped onion on one side of tortilla, fold, and continue frying until cheese is melty. Repeat with remaining tortillas. Top tacos with cilantro.