Season the leg quarters all over with salt and black pepper.
Heat the oil in a large skillet. When hot, add the leg quarters and onions and sauté for about 5 minutes, turning often.
Transfer the leq quarters and onions to a slow cooker, then add chicken broth and Sazon Goya seasoning and stir everything together. Place lid on the slow cooker and cook on Low for 8 hours or on High for 5 to 6 hours until meat is very tender and falling off the bones.
Remove the leg quarters from the slow cooker and transfer to a large platter or cutting board. Let the meat cool for about 10 minutes until safe to handle, then shred the meat using 2 forks. Be careful to remove and discard all the bones.
Heat a large skillet over medium heat. Cook tortillas for about 20-30 seconds on each side until soft and pliable. Remove tortillas to a plate.
Spoon about 2 Tablespoons of the cooked and shredded meat mixture and 1 Tablespoon of shredded mozzarella cheese into a line at one end of each tortilla and tightly roll it up. Use a toothpick to secure it. Then spray the tops of the tacos with cooking spray or brush them lightly with oil.
Bake the rolled tacos on a parchment paper-lined rimmed baking sheet at 425 degrees F for 15 minutes or until the shells are crispy.
Meanwhile, in a small bowl, whisk together sour cream and salsa verde and set aside.
Top rolled tacos with sour cream mixture, queso fresco cheese, and cilantro. Serve immediately with guacamole and pico de gallo if so desired.