Start by chopping the yellow onion, green bell pepper, red bell pepper, and jalapeño.
In a small bowl, mix together the chili powder, cumin, salt, black pepper, onion powder, garlic powder, and cayenne pepper (if using). Rub this spice mixture all over the flank steak.
Place the seasoned flank steak at the bottom of the slow cooker. Top it with the chopped onion, green bell pepper, red bell pepper, and jalapeño.
Cover the slow cooker and set it to low for 8 hours or high for 4 hours.
Once cooked, remove the flank steak from the slow cooker and let it rest. Shred the beef into bite-sized pieces and return it to the slow cooker.
In a dry skillet, warm the corn tortillas for about 30 seconds on each side.
Assemble your tacos with a warm tortilla, a generous amount of the beef mixture, and top with sliced avocado, cilantro, salsa, and lime juice.
