Cut the steak into two even pieces and place in a sealable plastic bag or container. Place the olive oil, coriander, basil, parsley, lime juice, garlic and one tablespoon of salt in a blender and mix until smooth. Reserve 120 ml of the herb-oil mixture and refrigerate. Add the remaining mixture to the meat, distribute evenly and marinate in the refrigerator for at least 3 hours.
Preheat oven to 190°C / gas mark 5.
Slice the ciabatta loaf lengthwise and remove excess bread from inside, if desired.
Remove the meat from the marinade; discard marinade. Place on a paper towel, wiping off any excess marinade. Heat a griddle pan over medium-high heat and add one tablespoon of rapeseed oil. Once the pan is hot, add meat and cook until medium-rare, about 4 to 5 minutes per side, depending on thickness. Place meat on a cutting board and cover with foil. Repeat with other piece of meat, only adding more oil if necessary.
Place bread directly on oven rack and toast, about 5 minutes.
Heat remaining tablespoon of oil in the griddle pan over medium-high heat. Add onions and cook until just beginning to brown but still crisp, about one minute. Remove from heat and season with salt, to taste.
Take the reserved herb-oil mixture and brush the inside of both halves of bread. Slice steak thinly on the bias and place on the bottom half of the bread. Top with onions and then remaining bread.
Slice bread into quarters and serve.