The title says it all really - par boiled onions, bathed in seasoned cream and parmesan, baked until bubbly and divine and the cream thickens to a gorgeous rich sauce. It's a Nigel Slater recipe, and the man does know what he is doing. Although, bizarrely, there is no mention of nutmeg which is just wrong really. Of all things, onions, cream and parmesan love nutmeg. So I added a good grating of fresh nutmeg when I seasoned it.
With all that divine decadence (™ Sally Bowles), you only need very simple (not to say bland) flavours alongside. A roasted chicken, with lots of garlic, a big pile of peas. You don't even need butter on the peas, and it is not often I say that!