Combine the marinade ingredients in a non-reactive bowl large enough to accommodate the steak and mix well to dissolve the sugar. Submerge the steak in the liquid and let it marinate for 2 days, covered, in the refrigerator.
When you are ready to cook, remove the flank steak from the marinade and let it come to room temperature, about 30 minutes. Heat a grill or grill pan on high heat. Rub the steak with 1 tablespoon olive oil. Grill the steak for about 8 to 10 minutes per side for medium rare. Let the steak rest for about 15 minutes before slicing.
While the steak is cooking and resting, heat the remaining olive oil in a skillet over high heat. Remove the onions and pears from the marinade and saute them until they are golden and caramelized, about 8 minutes. Set the onions and pears aside and strain the marinade into the hot skillet. Cook until the marinade is reduced by half.
To serve, slice the steak into thin slices against the grain. Top the slices with the caramelized onions and pears and the reduced marinade.
This recipe was provided by a chef, restaurant or culinary professional. It has not been tested for home use.