Position rack in center of oven; preheat to 425°F. Line a rimmed baking sheet with parchment paper.
Use a mortar and pestle to pound peppercorns to a semi-coarse texture. Transfer to a small bowl and mix in garlic powder, sugar and salt. Set next to the stove.
Lightly crush coriander and cumin seeds with the mortar and pestle. Transfer to a large skillet, add oil and cook over high heat, stirring frequently, until fragrant, about 1 minute. Add chile flakes and cook until the spices smell toasty, 5 to 10 seconds. Remove from heat and add potatoes. Gently stir to coat, then gradually stir in the pepper mixture. Transfer to the prepared baking sheet, spreading the potatoes in an even layer.
Roast the potatoes, stirring halfway through, until golden brown and tender, 30 to 35 minutes. Let cool for 5 minutes. Gently stir in herbs. Serve immediately.
