Heat the oil in a large saute pan over medium-high heat until hot, then add the shishito peppers. Cook, stirring frequently to allow even charring, until softened slightly and blistered in spots, 3 to 5 minutes.
Meanwhile, stir together the chili powder, sesame seeds, zest, 2 teaspoons salt and some black pepper in a small bowl. Combine ponzu and lime juice in another bowl.
Place peppers in a large bowl and toss with the spice mixture and about half the ponzu-lime mixture, stirring well to evenly coat the peppers. Place on a serving platter, finish with flaky salt and serve with the remaining sauce alongside.