Heat the avocado oil in a skillet over medium-high.
Add the ground beef, breaking it into small pieces. Season with salt and pepper.
Cook 4 to 5 minutes, until deeply browned and crispy in spots. Leave it undisturbed for the first couple of minutes. You want a crust, not a crumble.
Add the cumin and chili powder. Toss to coat and cook 30 seconds more, until the spices bloom.
Heat the tortillas in a dry skillet or straight over a gas flame until charred in spots and pliable. If you are using corn, double them up.
Fill the tortillas with the spiced beef.
Top with diced onion and cilantro, and squeeze lime over the top. Crema if you want it.