1 large shallot, sliced into very thin rings (about 1/3 cup)
3 tablespoons lime juice from 2 limes
4 teaspoons packed brown sugar
1 1/2 teaspoons kosher salt
1/2 teaspoon ground white pepper
1 1/2 pounds skirt steak, trimmed and cut into 2 to 3 pieces
3 tablespoons fish sauce
1 teaspoon red pepper flakes
1 1/2 cups (about 7 ounces) red or yellow cherry tomatoes, halved
1/2 cup coarsely chopped fresh cilantro, plus cilantro sprigs for garnish (optional)
1/2 cup coarsely chopped fresh mint
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 sugar, the salt, and white pepper. Pat the steak dry with paper towels, then rub all over with the sugar-salt mixture.
Prepare a grill 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 an even, high flame. Heat the grill until hot, about 5 minutes, then clean and oil the cooking grate. Grill the steak (directly over the coals, if using a charcoal grill) until charred all over and cooked to desired doneness, 2 to 4 minutes per side for medium-rare (depending on the thickness of the steak). Transfer the steak to a carving board and let rest for 10 minutes.
Meanwhile, add the fish sauce, pepper flakes, and remaining 2 teaspoons of sugar to the shallot-lime juice mixture and stir until the sugar has dissolved. Thinly slice the steak against the grain, then transfer to the bowl along with any accumulated juices. Add the tomatoes, cilantro, and mint and fold to combine. Transfer to a platter, garnish with cilantro sprigs, if desired, and serve.