Make the cassava flour tortillas. These can be made ahead of time and frozen, then reheated when ready to use.
Add all marinade ingredients to a bowl and stir. Pour the marinade into a ziploc bag (or shallow, covered dish) and add the steak. Marinade for at least 4 hours.
Heat a grill to medium high heat. Add the steak and cook for 4 minutes each side. Let the steak rest on a cutting board for several minutes before cutting.
While the steak is resting, heat the oil in a skillet (this is the one I use) on medium-high heat. Add the bell pepper slices and cook, stirring frequently for 4-5 minutes. Add the onion slices and cook for another 2-3 minutes.
Slice the steak into long, diagonal strips.
Assemble the fajitas by adding steak, grilled vegetables, slices of avocado, fresh cilantro and crumbled goat cheese to a tortilla.