Position racks in the upper and lower thirds of the oven and preheat to 450 degrees F. Line 2 rimmed baking sheets with foil, then divide the fries between them. Bake, tossing once or twice, until crisp and golden brown, 30 to 45 minutes. Transfer to a large bowl.
Place the chiles in a bowl, cover with a plate and shake to release the seeds. Fish out the chiles, leaving the seeds behind.
Heat the vegetable oil in a large skillet over medium heat. Add the cumin seeds, peppercorns, chopped chiles and 1 teaspoon salt and cook, stirring, until toasted, 2 to 3 minutes. Stir in the scallions and remove from the heat.
Toss the fries with the spiced oil and season with salt. Remove and discard any large chile pieces.