Step 1

Mix scallions, lemongrass, ginger, and garlic in a medium bowl. Heat oil in a small saucepan over high until hot but not smoking, about 2 minutes. Pour over scallion mixture. Let sit, stirring often to keep aromatics from burning, 5 minutes (scallions will sizzle, turn bright green, and wilt almost immediately). Mix in Aji-No-Moto seasoning (if using) and season with salt.

Step 2

Do ahead: Sauce can be made 2 days head. Cover and chill.

Step 3

Toast coriander seeds and cumin seeds in a dry small skillet over medium heat, shaking often, until spices are very fragrant and slightly darkened in color, about 3 minutes. Transfer to a spice mill or mortar and pestle and let cool. Finely grind, then transfer spice mix to a small bowl.

Step 4

Place chicken, skin side up, on a cutting board. Using your palms, press firmly on breastbone to flatten breast; you may hear a crack. This means you’re doing it right. Set chicken, skin side up, on a large rimmed baking sheet. Season generously on both sides with salt, then sprinkle spice mix all over, getting in every nook and cranny. (You may have a little spice mix left over.) Tuck wings underneath the breast. Chill, uncovered, at least 4 hours and up to 2 days.

Step 5

Let chicken sit at room temperature 1 hour before grilling. Drizzle with oil and pat all over.

Step 6

Prepare a grill for medium-high indirect heat (for a charcoal grill, bank coals on one side of grill; for a gas grill, leave one or two burners off). Place chicken, skin side down, on grate over indirect heat. Cover grill, placing vent (if your grill has one) over chicken so it draws heat up and over it. Grill until skin is lightly browned, 15–20 minutes. Turn chicken over and cook, still covered, until skin is deep golden brown and crisp and an instant-read thermometer inserted into the thickest part of breast registers 160°, 20–25 minutes. Transfer chicken to a cutting board and let rest at least 15 minutes before carving. Serve with lemongrass sauce and rice.