Firstly, this is a simple soup to put together - you just need onions, stock and mustard. Secondly, it's a great way of using up extra onions you might have left over. I first made this soup when I opened the fridge the day before food shopping to find a jugful of beef stock and some onions rattling about in the fridge drawer. I've made it many times since, and it's one of my favourites.
Roasting the onions first gives them a sweetness that's quite mellow in the finished soup, and although I've tried this recipe using both vegetable and chicken stock, I do think the beef stock works best, beef and onions being a classic combination. Perfect served alongside some cheese on toast (I've sprinkled mine with chives) and if you have any fresh thyme leaves about, then by all means sprinkle some of those into the soup too.
750g onions - red, white or a mixture of both
2 tsp duck fat (or olive oil)
salt and pepper
500ml good beef stock
half a tsp Dijon mustard
Get your oven pre-heated to 200ºC. Drop the duck fat onto a non-stick baking tray and slide into the oven as it warms up, to melt it down. After a couple of minutes, remove from the oven.
Peel and halve the onions, and then cut the halves into slices. Arrange these onto the baking tray and season with salt and pepper. Once you're oven is preheated, cook the onions for 30 minutes, until golden and just beginning to crisp up at the edges.
Tip the roasted onion slices into a medium-sized saucepan, and put on a medium heat. Pour in the beef stock and stir in the mustard. Simmer for about 10 minutes. Use a little of the stock to de-glaze the baking tray of caramelised onion bits before pouring into the saucepan.
Using a stick blender, blend the soup - I like it with a few chunky pieces of onion in, or if you like blend it completely. Taste, adding more seasoning or mustard, or a drop of water from the kettle if you want to thin it a little - and serve hot, with bread or cheese on toast.