Heat a gas grill to medium or light a charcoal grill and let it burn until the coals are medium-hot and covered with gray ash.
Lay the halved tomatillos cut-side down on a rimmed baking sheet or metal baking pan and slide the sheet onto the grill. Brush the ramps (or green onions or garlic chives) with oil. Lay them directly on the grill, along with the Serrano or jalapeño (no oil needed). Grill the ramps (or their stand-ins) and the chile, turning occasionally, until soft and richly browned – the ramps will take 4-5 minutes, the chile about 10.
Cook the tomatillos about 3 to 4 minutes, until soft and browned on one side, then flip them over and cook on the other side. Cool everything.