Preheat the grill or grill pan to medium high heat. Prepare a section of the grill for indirect heat or preheat the oven to 400 degrees F.
Dry the rib eye very well with paper towel. Season both steaks evenly on both sides with the salt, harissa powder and cumin seeds. Rub the seasoning into the meat. Drizzle evenly with the oil and place over medium high heat. Allow the steak to cook undisturbed until deep golden brown on the first side, about 3 to 4 minutes. Flip the steaks and continue to cook for 4 minutes on the second side. Check the temperature. You are looking for around 135 degrees for medium. Depending on the thickness of your steak it may already be there but if not, move the steak to indirect heat and cover it or to the oven and cook until it reaches the desired temperature, up to an additional 10 to 13 minutes.
Remove the steak to a plate or board to rest and squeeze the juice of the lemon half over the two steaks. Allow them to rest for 5 minutes before slicing and serving with a sprinkling of flake salt if desired.