Rip off the cilantro leaves. Remove the zest from the limes with a fine grater, squeeze the juice from the pulp. Peel the hot pepper from seeds and partitions and finely chop. Peel and finely chop the ginger and shallot.
In a bowl, grind the sugar with the zest, add the lime juice, cilantro leaves, hot pepper, ginger, shallots and fish sauce. Stir.
Cut 6 pieces of foil and form 6 boats out of them. Put one steak in each, spread the marinade on top. Close the boats and leave to marinate for 1 hour.
Heat up a grate over the coals. Place foil boats on a wire rack and bake for 15-20 minutes. Unfold the foil, check if the fish is baked. Serve immediately.