To make the taco seasoning: In a small bowl, whisk together the chili powder, cumin, sea salt, black pepper, garlic powder, onion powder, oregano, paprika, and red-pepper flakes (if using) until combined. You can also put all of the ingredients in a jar, cover it, and shake it until they are combined. Store in an airtight container for up to 6 months.
To make the tacos: In a resealable plastic bag, combine the chicken and taco seasoning. Shake the sealed bag to coat the chicken with the seasoning.
In a baking dish large enough to hold the chicken in a single layer, pour the lime juice. Arrange the chicken in the dish and bake for 15 to 20 minutes, or until a thermometer inserted in the center registers 165ºF and the juices run clear.
Let the chicken rest for 5 minutes before slicing into bite-size pieces. Divide the lettuce, tomatoes, scallions, avocado, and chicken among the tortillas. Serve with hot sauce on the side, if desired.