How to Make It
Heat 2 tablespoons oil in a large skillet over high. Add 3 1/2 cups mushrooms, and cook, stirring occasionally, until mushrooms are deeply caramelized, 6 to 8 minutes. Transfer mushrooms to a medium bowl. Repeat browning process with remaining 2 tablespoons oil and remaining mushrooms.
Return all mushrooms to skillet over medium. Stir in sherry vinegar and fleur de sel, scraping browned bits off bottom of skillet. Cook until liquid is reduced by half, about 1 minute. Add 4 cups cream, and bring to a simmer, whisking occasionally. Cook, whisking occasionally, until gravy is thickened and reduced to about 3 cups, about 30 minutes. If gravy gets too thick or separates, whisk in up to 1/4 cup more cream, 1 tablespoon at a time, until desired consistency is reached. Whisk in tarragon, and serve.