In a small, dry skillet over medium-low heat, toast the coriander and cumin, shaking the pan or stirring frequently, until the cumin seeds darken and become very fragrant, about 5 minutes. Transfer to a spice grinder or mortar and pestle, add the peppercorns and fenugreek, and grind to a fine powder.
Combine the cilantro, lime juice, garlic, 1-inch scallion pieces, canola oil, salt, turmeric, and ground spice mixture in a blender or food processor; blend into a smooth purée.
Put the chicken pieces in a large bowl, add the marinade, and turn to coat the pieces all over with the marinade. Set aside while you heat the broiler; or cover and refrigerate for up to 24 hours.
Position an oven rack about 8 inches from the broiler element; heat the broiler on high. Line a large rimmed baking sheet with foil. Arrange the chicken pieces, skin side up, on the baking sheet. If using a combination of parts, put the breast pieces on one end of the baking sheet and the thighs on the other end. Sprinkle generously with salt.
When the broiler is hot, broil until the chicken skin is brown and crisp all over, with some singed bits, about 20 minutes, rotating the pan occasionally for even browning. Check the chicken for doneness, either by cutting into pieces to see if they’re cooked through or by using an instant-read thermometer—breasts should be 165°F, thighs should be 170°F. If some or all of the chicken is still underdone by the time the skin is well browned, turn off the broiler and set the oven temperature at 450°F. Continue roasting the chicken, checking every 5 minutes, until cooked through, 5 to 15 minutes more. If some pieces finish earlier than others, transfer them to a platter and keep warm while you continue to roast the underdone pieces.
Arrange all of the chicken on a serving platter and garnish with the cilantro leaves, sliced scallion, and lime wedges.
The chicken can be marinated up to a day ahead.