Bistro onion soup
Four onions, sliced fine and sweetened the long way — a good half an hour on the low side, in a browned butter, until they are deep gold and soft — lifted by the white wine, built on a long stock, and topped with a slice of baguette under a browned mornay cheese. The whole flavor of the soup is in the first forty minutes, and none of it is hot.
Inspired by
The onion soup at the small Paris bistro, where the onions are sweated slowly on the low side before anything else touches them, so the finished soup tastes of caramel and the cheese crust, not of raw onion. The home version that turns the same four onions into a thin, raw-tasting pot of stock in twenty minutes skipped the one part that cannot be skipped: the time the onions are given. This page is that time, written out.
Technique notes
The browning is pan-heat at its low end — the onions sweat rather than sear, and the pan holds the heat while they go. The butter they sweat in is the un-salted end of the brown-butter scale, just past foaming, and the crust on top is the bechamel method with the cheese in — a mornay doing the job the bistro gives it on this plate.
Why the restaurant does it this way
A home kitchen can sweat an onion in six minutes and the soup still works — but it tastes of onion the way a raw onion tastes, and the bistro is not interested in that flavor. The onion's raw taste is the defect the method exists to remove, and the only tool that removes it is time on the low side: as the onion softens, its sugars caramelize, and that caramel — not the onion — is what the bistro means when it says onion soup. The wine goes in before the stock because it lifts the first caramel off the pan; the stock goes in long and slow because the soup needs a body, not a dilution. The mornay goes on a slice of baguette under the broiler because the crust needs a base that toasts into the soup instead of going soggy. The whole bistro difference is the same word three times: time on the low side.
Equipment
- A deep, heavy-bottomed pan wide enough to lay the onions in a roughly even layer — the pan holds the heat while they go
- Four heatproof bowls that will stand under a hot broiler, for the crust
Ingredients
- 800 g (about 4 onions) yellow onions, sliced 5–8 mm, the rings separated
- 45 g (3 tbsp) butter, plus a thin film of neutral oil — the oil keeps the butter from browning uneven
- 120 ml (½ cup) dry white, for the lift
- 1 L (4 cups) chicken or vegetable stock, long and slow
- 2 sprigs thyme, 1 bay leaf, salt and pepper to the taste
- 4 thick slices of baguette, for the base of the crust
- 120 g (4 oz) grated hard cheese — a comté, gruyère, or a mature cheddar — plus enough mornay to cover four slices
Method
- Melt the butter with the oil over the low side of medium, and add the onions with a pinch of salt. Turn them every few minutes, but do not chase them — the pan is doing the work, and the onions should be going from pale to gold to deep, soft gold. That takes 30–40 minutes, and it is the whole dish. When the onions are a deep even gold and as soft as a spread, they have done their part.
- Pour in the white and let it reduce by almost all — the caramel on the bottom of the pan is what the wine is lifting, and the lift is the last of the browning. That's 2–3 minutes.
- Add the stock, the thyme and the bay, and bring the whole thing to a gentle simmer. Run it with the lid off for about 15 minutes, until the soup coats the back of a spoon and tastes of the caramel, not the stock. Taste, and add salt — the soup should be a little under-salted, because the cheese on top is coming next and it has salt in it.
- Make the mornay (bechamel, with the cheese folded in, as that page runs it). Ladle the soup into the four heatproof bowls, float a thick slice of baguette in each, and pour the mornay over, spreading it to the edge of the slice.
- Run the bowls under a hot broiler for 3–4 minutes, until the mornay is puffed and a deep, spotty brown. The crust needs the slice under it — the slice toasts into the soup while the mornay bakes on top of it, and a crust with no base under it turns to a puddle. Serve while the crust is still standing.
Common failures
- A thin, raw-tasting soup that tastes of onion the way a sliced onion tastes. The onions went into the stock before they caramelized. The raw onion is the defect the method exists to remove, and the only tool that removes it is time on the low side. Give the onions the 30–40 minutes they are owed and the finished soup carries the caramel, not the onion.
- A crust that turned to a puddle before it reached the table. The mornay went on with no base, or the slice went in after the soup cooled and drank it all. The baguette slice goes in while the soup is still hot and the mornay goes on while the slice is still floating — the slice toasts into the soup while the crust bakes on top of it, and the two together are what a crust is.
- A soup that's a soup on top of a broth underneath. The stock went in hot and fast instead of long and slow, and the body never built. Simmer with the lid off for 15–20 minutes after the stock goes in — the reduction is what makes a body out of the stock, and the soup should coat the back of a spoon.
Variations & substitutions
- A vegetable stock version, for the table. The method is the same, and the vegetable stock does the job the chicken stock does — the stock is the body, whichever one it is.
- More cheese, less bread, for the gratin version. Skip the baguette, put a little more mornay in each bowl, and run them longer under the broiler until the top is a solid, deep-brown crust across the whole surface. The soup loses its base and gains its crust, and the finished bowl eats a little denser and a little richer.
- Half the onions, for a quicker weeknight version. Four onions still need the 30–40 minutes they are owed — time is the one part of the method that halves don't fix. The weeknight move is the mornay kept in a pot and the onions sweated while something else is cooking, so the time is the dinner's, not the soup's.
What to serve with it
Ahead of a grilled ribeye, as the bistro puts the course — the soup is the first course, and the steak is the one it warms up to. After, a green salad with a lemon-oil vinaigrette cuts the richness of the mornay. And the baguette is already there, in the crust, where the bistro leaves it.
More
Back to Restaurant · Techniques (pan heat, brown butter, bechamel)