French onion soup

Brine the marrow bones. This thaws them if frozen, deeply seasons the marrow, and draws out hemoglobin, blood and impurities. It also bleaches the bones whiter.

Invert the bones cut side down on a tray, crinkle aluminum foil up into the negative space around them and invert again, so the foil holds each bone in its own crater and catches the fat.

Roast at 400F for about 20 minutes, until the marrow is a little puffy and the fat is starting to render. 450F is normal, but 400F is enough when the marrow is only being harvested.

Peel and halve the onions: cut off the tip, cut in half, and strip off the whole outermost layer, which is tough and dried out. Slice the roots off so the flimsy bits do not end up in the dish.

Remove the onion's stem, which is tough and stops the layers separating, then slice the onion holding it in a claw, going at an angle and turning it when it gets tall and unwieldy.

Slice the baguette about two centimetres thick with a straight edge bread knife, choosing the wider pieces that fit the crock and avoiding slices with a big air bubble, since cheese falls through a hole instead of sitting on a raft of bread.

Cut some of the toasts into quarters so they are bite-size in soup that is too hot to cut in the bowl.

Lift the marrow out of the roasted bones and separate the marrow from the liquid fat.

Paint the liquid marrow fat onto the bread on both sides rather than drizzling it, so the coverage is even and no patch stays pale.

Salt the toasts lightly. Layering seasoning throughout means every bite is seasoned rather than needing to be mixed with something else to taste.

Toast on a rack on convection at 400F, checking often, since toasting time in an oven depends on slice thickness, bread and hot spots.

Trim any dried rind off the Gruyere and grate it coarsely along the full length of the piece, so the strands are long enough to prop up on the bread instead of sinking into the soup.

Melt the butter in a non-stick 12 inch saute pan over medium heat until foaming, then add the onions. There should be barely any sizzle; too much heat browns them before the sugars develop.

Add about half a cup of water, cover and reduce to medium low. The water stops the butter burning, stops it browning too fast, steams the onions down so they shrink and fit the pan, and acts as a mini deglaze for the fond.

Keep the onions moving as they shrink. Splash in water and scrape the fond up with the onions and the spatula whenever it sticks. Adding sugar is a shortcut, but then you are caramelizing added sugar rather than the sugars in the onion, and it is less sweet, jammy and complex.

Spread the onions out and push them up the walls of the pan: more surface area against the pan and less depth, so less steam builds and they brown more readily.

Finish with one last squirt of water around the outside, scraping everything up with the onions as a mop. They should be sweet, soft and jammy with no black spots.

Heat one tablespoon, maybe a tablespoon and a half, of butter in the soup pot and add the caramelized onions.

Cook off the added water and build fond on the bottom of the pot, chopping the onions down a little so there are no long strands hanging off the spoon.

Add about two tablespoons of flour and mix to a paste, keeping well shy of the bechamel rule of a tablespoon each butter and flour per cup of liquid, since this wants body not thickness.

Let the paste brown a little, keeping it golden rather than dark.

Add about two teaspoons of tomato paste and mix it into the onions and flour.

Deglaze with two to three tablespoons of brandy, raising the heat so the liquid gets hot fast enough to lift the fond.

Add a splash of stock and simmer to lift the rest of the fond, then add the remaining stock and bring to a strong simmer, mixing so the flour coating dissolves into the stock and gives it body.

Add the soy sauce, Worcestershire and fish sauce as glutamate boosters. Marmite was planned but left out as overdoing it. Do this before any salt, because the soy sauce is very salty.

Grate fresh garlic straight onto the toasts against the grater, which punctures the cell walls and coats them in garlic oil and shavings.

Whip the reserved marrow with a small whisk into a smooth spreadable paste and spread it on the top toasts just before they go in the soup.

Make a bouquet garni: a couple of sprigs of parsley, a couple of sprigs of thyme and a bay leaf wrapped in leek leaves and tied with butcher's twine, so the herbs can be lifted out in one package.

Drop the bouquet garni into the soup and lid it, so the flavours meld without the soup reducing and going thin.

Simmer 30 minutes in all.

Preheat the broiler and line the tray with foil, since cheese will spill down the outside of the crock and burn on.

Put cheese in the bottom of the crock first, then the croutons, so the cheese holds the floating bread down, then ladle in the soup.

Arrange the marrow-spread toasts flat over the top and cover with enough Gruyere to completely insulate the bread and the marrow so neither burns.

Grate Parmigiano Reggiano over the top, which is not French but helps the browning and the brown spots, and scatter a few shreds around the crock.

Chop the chives and garnish.

Broil, watching closely, until the cheese is melted with brown spots. Fish the bouquet garni out before serving.

Taste for seasoning right before serving, since the soy sauce, Worcestershire and the broth all bring their own salt.