Score the top and bottom of the flank steak. Mix all ingredients in a gallon zip lock bag. Release as much air as possible from the bag and let marinate overnight. Remove steak from marinade and season with salt and pepper. On a preheated grill, grill over high heat 3 minutes per side for medium rare. Remove and let rest for 5 minutes.
For the prawns, combine all ingredients in a zip lock bag and shake to coat. It can marinate for up to 1 hour but no longer. Remove the prawns from the marinade and place on preheated grill, turning only once, until curled and pink. Serve with the steak and salad.
For the salad:
Combine all ingredients, except the lettuce, tomato, and onion, in a bowl and whisk well to incorporate. Season with salt and pepper. Slice the heads of romaine lengthwise and brush with a little olive oil. Place on a preheated grill and grill the lettuce, moving it around carefully, until charred and nice grill marks form. Remove the lettuce and tear into pieces. Place in a bowl with the onion and tomato and drizzle with the dressing. Serve with the steak and prawns.
Using a plate with a wide lip, place the grilled salad in the center. Then plate slices of steak and prawns alternating around the lip.