Balsamic vinegar was one of my dad’s very favorite foods, so when I brought these balsamic-glazed mushrooms to Thanksgiving dinner at my sister’s house one year, I knew they would have at least one fan. If I were leafing through a book and saw this recipe, I might be turned off, thinking that a mushroom drenched in vinegar would be overly sweet, and an unappealing black color. And although they wouldn’t win any beauty contests, these mushrooms are neither drenched nor overly sweet. They do indeed turn out black — not the most appetizing color in a vegetable, I admit — because the mushrooms are cooked whole, the little black globes are pretty in their own way. Both their look and flavor are brightened up with fresh tarragon leaves that are tossed with the mushrooms after they’re cooked.
When shopping for the mushrooms for this dish, choose those that are firm, with unopened caps (you don’t see gills under the caps). When choosing white button mushrooms, pick those on the larger end of the spectrum.
Divide the balsamic vinegar in half so you have two 1/2-cup containers.2.
Cut the shallot in half root to tip; trim and discard the root end. Separate the layers of the shallot, stack two or three layers at a time on top of one another, and slice 1/16-inch thick lengthwise.3.
Heat 2 tablespoons of the olive oil in a large sauté pan over medium-high heat until it slides easily in the pan and the oil around the edges of the pan begins to smoke, 2 to 3 minutes.4.
Add half of the mushrooms, caps down, to the pan. Season with 3⁄4 teaspoon of the salt and cook the mushrooms for about 10 minutes, until they are golden brown, turning them as they brown and adding 2 tablespoons of the remaining oil, 1 tablespoon at a time, as the pan becomes dry.5.
Pour one-third of one portion of the balsamic vinegar into the pan and cook the mushrooms for 3 to 4 minutes, shaking the pan and turning the mushrooms to coat them evenly with the vinegar, until the mushrooms have absorbed the vinegar.6.
Add half of the remaining vinegar from the same portion you’re working from and cook the mushrooms in the same way, shaking the pan and turning the mushrooms to coat them evenly until the newly added vinegar has been absorbed, 3 to 4 minutes.7.
Add the remaining vinegar from the portion you’re working from and cook until the vinegar left in the pan is syrupy and caramel-like, but not overly thick and sticky, 2 to 3 minutes. (Lift the pan off the heat so the bubbles subside and you are better able to see the consistency of the vinegar.) If the vinegar is over-reduced to a sticky mess, add another tablespoon of vinegar and cook it for about 1 minute, or just long enough to integrate it with the balsamic in the pan.8.
Turn off the heat and add half of the shallot slices to the pan. Gently stir to combine the shallot slices with the mushrooms and to coat the shallot with the balsamic glaze.9.
Transfer the mushrooms and shallot slices to a large bowl. Cook the remaining mushrooms in the same way, using the remaining 1/4 cup oil, 3/4 teaspoon salt, and the second portion of the vinegar, and adding the remaining shallot slices at the end as you did when cooking the first batch of mushrooms. When the second batch is done, add it to the bowl with the first batch. Sprinkle the chopped tarragon over the mushrooms and shallot slices and toss gently to combine.10.
To serve, pile the mushrooms and shallot slices, making sure the mushrooms are cap side up, 2 to 3 high, in a large shallow bowl or a rimmed platter. Scatter the whole tarragon leaves over the top and serve with a large spoon.