Preheat oven to 400°F. Arrange onions on a rimmed baking sheet and roast, shaking baking sheet halfway through, until the innermost layers of largest onions are just tender (some of the onions will be jammy and others will have some bite to them), 3040 minutes. If you have one, a cake tester is a great tool to check doneness. Transfer onions to a clean surface and let cool slightly. Halve through root ends and remove papery layers. Separate layers into individual petals and transfer to a platter or a large bowl. Drizzle with oil and vinegar and toss gently to coat; season with salt and pepper.