01/16/2013 09:54 am ET Updated Mar 18, 2013

Cooking Off the Cuff: Very Oniony-Mushroomy Soup

People always say you shouldn't serve recipe experiments to guests, but I do it from time to time - at least, when I've got enough spaghetti in the cupboard or eggs in the fridge to ensure a quick backup dinner in the event of disaster.

One recent evening, the main course was indeed to be something of an unknown (a new technique for cooking corned beef, the details of which I shall spare you for now), so, besides having the makings of an emergency omelette to hand, I also felt I ought to offer a particularly delicious first course that wasn't too loony. A number of days earlier I'd cooked a few pounds of sliced onions in butter until tender but not mushy, and I had a good two cups of these left. I could have made them into a tart (this one perhaps) , but then I saw my London buddy Angela Harnett's column on The Guardian newspaper's on-line food page. It was about a pureed onion soup scented with thyme and garnished with flaked fish, which sounded delicious. And the onions were already cooked, always a plus.

But, having adopted Angela's idea of a smooth onion soup, I didn't really want more meat - even that of a fish - in our beef-centric dinner. Maybe, I figured, a garnish of herbs and crisp croutons would be enough, and that became the imperfect but acceptable plan.

Then, on a shopping excursion to New York's Eataly, I saw beautiful (cultivated) pioppino mushrooms with dark brown caps on pale stalks: far prettier than the lighter-colored caps you sometimes see in more mature specimens. They have a distinctive woodsy flavor and aroma and would a real asset, both in the body of the soup and as a garnish.

Turning these things into soup was a cinch. I warmed those two cups of cooked onions in a saucepan and added a little more than two cups of chicken stock; vegetable broth would have been good here too. As it came up to the simmer, I trimmed the mushrooms. Like many other mushrooms that grow on wood, pioppini come in little clumps with dense, sometimes dirty, bases. From these, I cut the mushrooms free and set them aside, then trimmed dirty parts away from the bases. I roughly chopped the cleaned bases and added them to the pan with the onions and stock, along with a few fresh sage leaves (another herb would have been fine: thyme, maybe, or even parsley - but if you're using parsley use a lot of it). After just a few minutes of simmering, I pureed the mixture in a blender (better than a food processor if you want a very smooth soup) and set it aside until dinner time.

The mushrooms I rinsed very, very briefly - in and out of a big bowl of water, then straight into a hot pan with sizzling butter. Once the residual water had mostly evaporated, I lowered the heat to medium, added three or four slivered sage leaves and salt and pepper, and sauteed the mushrooms until cooked but not too soft. I set them aside.

For the croutons, I used a fairly sturdy white bread (this one, but in pan loaf form), crusts removed, torn into pieces to create craggy shreds of bread. I fried them, with three whole sage leaves, over lowish heat, using olive oil and a little butter for flavor, tossing frequently, until they were crisp. This took a long time, at least a dozen minutes, and needed fairly constant attention to prevent burning. These too I set aside.

At dinner time, I put the croutons and the mushrooms into little pans over low heat to warm through; I heated the soup, added about a quarter cup of cream and another half cup of stock (this was to adjust consistency: use your judgment). I checked for seasoning, which, curiously, was just right. A six-ounce ladleful of soup went into each warm bowl, along with a scattering of mushrooms and croutons.

It was clearly an onion soup, and it was clearly a mushroom soup: All the key elements, including the sage, were there for the tasting. And those pioppino mushrooms were a real hit; in fact, we went back to Eataly the next day and bought some more.

Very Oniony-Mushroomy Soup