Rub the pork shoulder evenly with the ground cumin, dried oregano, chili powder, sea salt, and black pepper.
Place the seasoned pork into a slow cooker along with the smashed garlic cloves, orange juice, and lime juice.
Cover and cook on low for 8 hours or high for 4-5 hours until the meat is tender and easily shreds with a fork.
Remove the pork from the slow cooker and shred it using two forks, discarding any large pieces of excess fat.
Heat the avocado oil in a large cast-iron skillet over medium-high heat.
Add the shredded pork to the skillet in a single layer and press down with a spatula, searing for 3-4 minutes until the edges are crispy and golden.
Warm the corn tortillas in a dry pan or over an open gas flame until soft and slightly charred.
Divide the crispy pork carnitas between the tortillas and garnish with finely diced red onion and fresh cilantro.