Preheat a grill pan. In a small bowl, toss the scallions, garlic and jalapeńo with the vegetable oil. Grill over moderately high heat, turning occasionally, until charred all over, 5 to 6 minutes. Transfer to a work surface and let cool.
Finely chop the scallions and jalapeńo and transfer to a medium bowl. Peel the garlic cloves, mash them to a paste and add them to the bowl. Scoop the avocado flesh into the bowl and coarsely mash with a fork. Fold the cilantro and lime juice into the guacamole, season with salt and pepper and serve.