1. Prepare a grill or heat the broiler. Trim roots and ends from 20 spring onions, leaving about 8 inches of green stalk. In a large bowl, toss onions with 1 tbsp vegetable oil or mild olive oil and 1/2 tsp salt.
2. Grill onions on heated grill, turning them with tongs 3 or 4 times, for 5 to 7 minutes or until softened and lightly charred. If you don't have a grill, broil onions on a broiler pan about 3 inches from heat, turning 2 or 3 times, for 10 to 12 minutes. Transfer onions to a platter and squeeze 2 lime halves over them.