Prepare an outdoor grill or stovetop grill pan.
Cut about 2 inches of the green ends off the onions, and trim the shaggy root ends, leaving a tiny bit to hold the onion together.
Brush the onions all over with vegetable oil and place them on the hot grill.
(If the green part starts to burn, put a piece of foil under them.
) Cook the onions until tender, turning once or twice, about 6 to 8 minutes.
Sprinkle with lime juice and season with salt.