In a small skillet, toast the salt and peppercorns over medium heat until fragrant and the salt has browned slightly, about 10 minutes. Remove the skillet from the heat and let the spice mixture stand until cool, about 15 minutes. Using a mini food processor or spice grinder, process until finely ground. Reserve the skillet.
Fill a large Dutch oven with enough oil to reach a depth of 1 inch; heat over medium-high heat until it registers 350 degrees on a deep-fry thermometer. In a small bowl, whisk together 1 teaspoon of the spice mixture and the flour. Spread the chicken in an even layer on a baking sheet and sprinkle the flour mixture on top; toss to combine. Working in 2 batches, fry the chicken, stirring occasionally and allowing the oil to return to 350 degrees in between batches, until golden, 8 to 10 minutes; drain on paper towels. Sprinkle immediately with some of the spice mixture. Transfer to a platter.
In the reserved skillet, heat 1 tablespoon of the frying oil over medium heat; add the garlic and cook until just golden, about 2 minutes. Add the serrano chile and cook until crisp-tender, about 2 minutes. Spoon over the chicken and serve immediately with the remaining spice mixture.