Step 1

Preheat the oven to 375*F/190*C/gas mark 5.  Have ready a baking dish that you can fit all the onions in upright, snugly.

Step 2

Carefully trim the bizarre bits from the root end of the onion, leaving the root and skins intact.  Trim a sliver from the other end as well. Make a cross cut into each onion 3/4 of the way down the height of the onion, taking care not to cut all the way to the bottom.

Step 3

Rub the onions all over with the oil, season with salt and pepper and then fit snugly into a small ovenproof dish.  Cover tightly with aluminum foil.

Step 4

Roast for 70 minutes.  Uncover.  Divide the butter amongst the onions, sliding it down into the centers.  Bake for a further 25 minutes, basting them with the melted butter a few times.  Serve at once.