Whisk together the lime juice, scallions, garlic, cilantro, oil, red pepper flakes, coriander.
Place the meat in a large dish, flat. Pour the marinade over the steak, cover, and refrigerate for 8 to 12 hours, flipping 1 or 2 times.
Heat the oil (or lard) in a large non-stick skillet. Add the onions.
Cook for 15 minutes, stirring occassionaly
Add the soy sauce and cook for another 20 minutes, caramelizing the onions. They should turn soft and and a nice brownish color. Set aside.
Mix together all ingredients of the fajita seasoning in a medium bowl.
Prepare a medium-hot grill.
Remove the steak from the marinade and liberally sprinkle the seasoning all over the meat.
Use tongs to place the seasoned meat over direct heat. Cook until lightly charred on the outside, but still pink on the inside. Approx. 4 minutes per side. Internal temperature should be between 135°F and 145°F.
Bring in the steak and cut into 1/4 inch strips against the grain.
Place your fajita pan (or sturdy skillet) on the stove over medium high heat to heat completely. About 4 to 5 minutes.
Add the caramelized onions to the hot pan (be careful...they will sizzle!)
Then place the meat on the onions. Bring to the table and squeeze fresh lime over the fajitas.
Serve with warmed tortillas and all the fixin's.