Anyone who’s had grilled-meat tacos al carbon has probably been asked if they want a side of cebollitas (sometimes called cebollitas chabray), grilled knob onions. With a sprinkling of salt and a squeeze of lime, their uniquely light sweetness is the perfect accompaniment. You can grill common grocery-store green onions in the same way as the Mexican grocery knob onions, but the eating experience is very different. Most green onion varieties grow straight, never bulbing at the white end. Knob onions are actually tiny white onions, picked when the bulbs have reached about an inch in size. The bunch and knob size vary greatly. A 5- to 7-ounce bunch seems pretty common these days, but you’ll have to adjust to what you find.
Heat a griddle or gas grill to medium, or build a charcoal fire and let it burn until medium hot. Brush or spray the knob onions (all parts) with oil or lard. Sprinkle liberally with salt. Lay the onion knobs on the hot griddle or grill, the green tops away from the hottest part. (To protect the greens from burning, I usually slide foil under them.) Turn the onions regularly until the knobs are browned and their texture has transformed from crunchy to softening—all the way to their centers. Serve with the lime wedges for everyone to squeeze on al gusto.