Sea Salt Rib Roast

Preheat oven to 425°F with oven rack in lower third of oven. Rub roast all over with sea salt and place roast, fat cap up, on a wire rack set inside a roasting pan. Let roast stand at room temperature while oven is preheating, about 30 minutes. (Roast may be seasoned and refrigerated, uncovered, up to overnight. Let stand 30 minutes at room temperature before proceeding.) Insert a probe thermometer in thickest part of roast.

Roast in preheated oven until meat is lightly browned, 20 to 25 minutes. Without opening oven, reduce oven temperature to 225°F. Continue roasting until internal temperature of meat registers 120°F, about 2 hours and 30 minutes for medium-rare. Transfer roast to a cutting board and let rest 40 minutes before slicing.

Meanwhile, when the roast has about 30 minutes left in the oven, prepare the gravy. 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 the rest of the 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. Keep the cream gravy hot until ready to serve, up to 30 minutes

After it's rested, slice the roast and sprinkle with sel gris. Whisk the tarragon into the gravy, then serve with the rib roast and carving board juices.