This is another good recipe from Elizabeth David, satisfying in its simplicity. It is so straightforward that we quote her in full rather than numbering what would be artificially delineated steps.
“This is one of the most primitive but at the same time most appreciated of onion dishes.
Choose medium to large onions, but not monsters, and see that the ones you buy are clean, without grit showing through the skins. Do not peel them.
Put them on a baking sheet or in a tin and let them cook for 2 to 3 hours ... in a very moderate oven, gas No. 3. 350° F.
Serve them hot, exactly as you would baked potatoes, with butter, or cinnamon butter, and salt.” (Salt, Spices and Aromatics in the English Kitchen, 130)
Notes: The onions are particularly good sides for any main dish that has a sauce, although David seems to consider them a stand-alone offering for lunch or a light supper; she says to allow two of them per person.
Cinnamon butter is a traditional English accompaniment to to roast or pan-fried fish; while the juxtaposition sounds strange, it is one of those pairings, like ham and cheese, that proves greater than the sum of its parts. David is right that cinnamon butter goes well with roast onions too, although take care not to let its assertive flavor clash with a main dish served with them. To make it, mash together 3 Tablespoons of butter and a good teaspoon of cinnamon along with a little cayenne and lemon juice. Chill this compound butter before topping your fish or onions with it.