In a large bowl, combine the shallots and lime juice and let sit for 10 minutes, stirring occasionally. In a small bowl, combine 2 teaspoons of the sugar, the salt and white pepper. Pat the steak dry with paper towels, then rub all over with the sugar-salt mixture. If using a cast-iron or carbon-steel skillet, cut the steak into 4 to 6 pieces if needed to fit into the pan.

Prepare a grill or skillet for very high heat. For a charcoal grill, spread a full chimney of hot coals evenly over half  of the grill bed. For a gas grill. set all burners to a high, even flame. Heat the grill for 5 minutes, then clean and oil the cooking grate. For a cast-iron or carbon-steel pan, heat 1 teaspoon canola oil over medium-high until smoking, about 5 minutes.

If using a grill, grill the steak (directly over the coals, if using a charcoal grill) until charred all over, 2 to 4 minutes per side. If using a skillet, sear the steak in 2 batches until charred, 2 to 4 minutes per side. Transfer the steak to a carving board and let rest for 10 minutes. 

Meanwhile, add 1 tablespoon of  the fish sauce, the pepper flakes and the remaining 2 teaspoons of sugar to the shallot-lime juice mixture and stir until the sugar has dissolved. Taste, then add additional fish sauce, if desired. Thinly slice the steak against the grain, then transfer to the bowl along with any accumulated juices. Add the tomatoes. cilantro and mint and stir. Transfer to a platter and garnish with cilantro sprigs, if desired.