Heat the avocado oil in a large skillet over medium-high heat.
Add the diced white onion and sauté for 3-4 minutes until translucent and soft.
Add the ground beef to the skillet, breaking it up with a spatula, and cook until browned and fully cooked through.
Drain any excess fat from the pan if necessary.
Stir in the chili powder, cumin, garlic powder, sea salt, and black pepper until the meat is evenly coated.
While the meat simmers on low, lightly toast the corn tortillas in a dry pan or over an open flame until warm and slightly charred.
In a small bowl, whisk the Greek yogurt with the lime juice to create a zesty crema.
Assemble the tacos by dividing the beef mixture between the tortillas.
Top with shredded romaine lettuce, diced tomatoes, and a generous dollop of the lime-yogurt crema.