Halve squash lengthwise and discard seeds and strings. Using outer ridges as a guide, cut squash lengthwise into sections. Arrange squash on a steamer rack in a saucepan and steam over boiling water, covered, until tender, about 15 minutes.
While squash is steaming, in a bowl whisk together lime juice, oil, and salt and pepper to taste.
On a platter drizzle squash with dressing and sprinkle with scallion