How to Make It
Put the serranos in a small heatproof bowl. In a small saucepan, bring 1/2 cup of water to a boil with the vinegar, sugar and 1/2 teaspoon of salt, stirring to dissolve the sugar. Pour the brine over the serranos and let stand for 15 minutes, until cooled to room temperature.
Meanwhile, in a small bowl, whisk the soy sauce, lime juice, ginger, mustard and 3 tablespoons of the oil.
In a large cast-iron skillet, heat the remaining 1/2 cup of oil until shimmering. Add the tofu and cook over moderate heat, turning, until crisp, 5 minutes. Using a slotted spoon, transfer to a paper towel–lined plate. Season with salt.