The excellence of onion soup is all about the ratio of its four main components – broth, onions, cheese, and croutons – and without the proper bowl, that ratio is inevitably out of sync.
– Thomas Keller
Sometimes it takes someone else to connect the dots. In this case a woman I’ve never met but am linked to through family and friendship and to whom I feel a kinship for her vegetable-forward ways. When I was deep in testing veggie broth characterised by papery onion peels, she was making a vegetarian take on French onion soup.
It prompted the thought: if it’s called Soupe à l’oignon, why not make it all about those brown onions – not just in the substance of the soup but also in the stock? Deep down I knew why. It may be a caramelised onion’s raison d’être, but the resulting sweetness is highlighted, held, and balanced by the deeply savoury quality of beef broth, which also lends a dark colour.
As coincidence would have it, the very reason for working on that veggie broth was to create more savoury layering in the flavour and it is defined by a rich brown colour – a magical transformation thanks to the onion skins. It’s not ideal for pale and creamy soups, which then turn a dirty beige, but in the case of French onion soup, is exactly what’s called for.
Onion soup happens to be a milestone along our cooking path, specifically one by chef Thomas Keller1. The process of caramelising a stockpot of onions for five hours is what opened our eyes to the full range of flavour nuance a caramelised onion can offer. It also illustrated how a bowl can entirely change the experience of eating and why there is one designed for purpose.
A French onion soup bowl has height, and a rounded belly to contain the soup. It narrows slightly to a mouth just wide enough for the croutes to cover the surface. And when grilled, the cheese laid over the raft of bread fuses with it and the edges, so there’s some anticipation in accessing what’s below – like breaking through the pastry on a pot pie to reach the filling.
The toasted baguette soaks up the broth like a sponge while keeping its basic structure and the bowl is sized to hold the right amount of soup in relation to it and the crust of melted cheese it’s carrying. So once the giant cheesy crouton is broken and submerged, the whole becomes a meal in its own right.
All of this makes enough of a difference for the broth to be more negotiable than the bowl. The veggie broth was gentler as it leaned into the sweet side of the onion but this is no two-legged stool without the beef. When built as above, it won me over as a robust lunch. One that doesn’t have its sights set on Paris but celebrates every layer of the onion – including the skin.
The bowl
To give an idea of dimensions, the bowl pictured is roughly 8cm deep (on a curve) from a 10cm wide mouth to a 5cm wide base (7cm on the outside). If not the exact bowl, perhaps it’s possible to come close with something of similar dimensions.
The soup
These quantities fill one soup bowl with the dimensions above, which could make it a dinner for one with a sense of ceremony. (The advantage of preparations for one is there’s as much enjoyment with far less onion slicing than if for four.) Expand quantities and cooking times as needed.
One serving is roughly 600g onions (weighed whole) to 550ml broth.
The first three steps can be done a day ahead of grilling and serving.
Caramelise the onions
Make the broth
Add together and simmer 1 + 2
Build, grill, and serve
Make this when time is on your side and you’re feeling full of calm, patience and curiosity. To enjoy the process, make the onions and broth one day ahead, and buy the baguette one day ahead too so it dries out. Peel close to 600g onions (about two to three brown onions) and save the skins and ends for making the broth. Slice the onions into even, elegantly slim slices. Melt two tablespoons (30g) unsalted butter in a medium pot over medium heat and swirl it around to cover the base.
Add the onions and 2.5ml (a half teaspoon) sea salt, stir well, replace the lid, turn the heat down to low to medium-low and caramelise for one hour (take the lid off after the first 15 minutes and stir regularly during the last 30 minutes). Keep a watchful eye as they can catch and darken in an instant, especially towards the end. If necessary, adjust the heat so it stays at a happy bubble. The aim is not waiting until the time is up, but gently coaxing out sweetness and golden brown colour. If necessary cook a little longer to get there. Add 1.25ml (quarter teaspoon) flour and cook briefly, stirring. Take the pot off the heat, replace the lid and set aside.
Make the broth alongside the caramelising onions, keeping an eye on them all the time. Apply the veggie broth method but keep it within the parameters of onion, leek, carrot, celery, celeriac, and soup celery.
Cut a square of muslin and wrap it around one bay leaf (fresh if possible), two black peppercorns, and one sprig of fresh thyme. Drop it in the onion pot, gradually add 550ml of vegetable broth, stirring, and then bring to the boil. Once it is boiling, reduce the heat to a simmer and cook for 20 minutes.
Taste and season with a little more sea salt if necessary and a little Sherry vinegar. Cut two 1.5cm thick slices from a day-old baguette, brush with olive oil and toast under the grill until golden on both sides.
The cheese
The original uses two slices of aged Comté or Emmental around 2mm thick and at least 10cm squared as the base, covered evenly with just over a quarter cup of the same, grated. We had better luck with overlapping narrower but uniform slices of locally made Gruberg (made in the style of Gruyère) than whole slices of Emmental as they weren’t quite big enough to have the corners drape over the sides and didn’t melt as well. But this all depends on what is available and it may be a combination of the two.
Ladle the soup into the bowl, lay the toasted baguette slices on top, cover with an even layer of cheese and grill until the cheese is bubbling and browned in patches. Enjoy.
Updated and edited on 10 July 2024.
Bouchon by Thomas Keller (New York: Artisan, 2004)