4 brown onions (about 1kg total)
2 tablespoons grapeseed oil or vegetable oil
Flaky sea salt
Preheat oven to 150°C. Halve onions through root ends (do not trim tops). Remove outermost papery layers from each onion.
Heat a large cast-iron skillet pan over high. Add oil and swirl to evenly coat skillet. (Oil should shimmer and start to smoke almost immediately.) Add onion halves, cut sides down, nestling them close together. Reduce heat to medium-low and cook, undisturbed, until cut sides are blackened, 18–20 minutes. Surfaces will look and smell burnt.
Transfer skillet to oven and roast onions until the innermost layers are no longer raw but are still rigid, 10–15 minutes. Remove skillet from oven and transfer onions to a large rimmed baking sheet, placing cut sides up.
Let onions cool slightly, then trim papery tops with kitchen shears. Separate layers into individual petals; discard root ends (some center pieces may not curve, which is okay). Arrange petals in a single layer and season with salt.