In a small bowl, mix together the salt, black pepper, garlic powder, onion powder, cumin, chili powder, and smoked paprika. Rub this spice mixture all over the flank steak.
Heat the olive oil in a skillet over medium-high heat. Sear the flank steak for about 2-3 minutes on each side until browned. This step is optional but adds great flavor.
Place the seared flank steak in the crockpot. Pour the lime juice over the steak and sprinkle the chopped cilantro on top.
Cover and cook on low for 6-8 hours or on high for 3-4 hours, until the meat is tender and easily shredded with a fork.
Once cooked, shred the carne asada with two forks and mix it with the juices in the crockpot.
Warm the corn tortillas in a dry skillet over medium heat for about 30 seconds on each side.
Assemble the tacos by placing a generous amount of carne asada on each tortilla. Top with diced onions, tomatoes, and avocado slices. Serve with lime wedges on the side.