Gather the ingredients.
Preheat an oven to 400 F.
Trim and peel the onions, removing and discarding their stem and root ends along with their papery peels. You can leave the smallest onions whole, but you will want to cut most onions into halves or quarters, depending on how big they are. Bigger than bite-size is fine, but you don't want diners wrestling with giant whole onions that are tricky to cut.
Put the onions in a large baking dish (they should easily fit in a single layer for the best, most even roasting).
Drizzle the onions with the oil and toss to coat them thoroughly and evenly. If you've cut them, arrange the onions to the cut-side is down, facing the pan (this will help them brown nicely).
Sprinkle the onions with salt (sprinkle from a few inches above the onions so the salt falls more or less evenly over them).
Bake the onions, turning them once or twice if you think of it until they're completely tender to the bite and browning on the edges - about 30 minutes total.
Drizzle the onions with the vinegar before serving, if you like. Start with a smaller amount of vinegar and add more to taste if you like the effect.
Serve and enjoy!