In a nonstick 12-inch skillet over medium-high, melt the butter. Add the onion, mushrooms and ½ teaspoon salt, then cook, stirring occasionally, until the liquid released by the mushrooms has evaporated, the onions are softened and the mixture browns, 12 to 15 minutes. Add the wine and cook, stirring, until fully evaporated, about 3 minutes. Transfer to a medium bowl and cool completely.

Meanwhile, in a large bowl, whisk the eggs. Add the crème fraîche and whisk until thoroughly combined. Add the cream, cayenne, nutmeg, ¼ teaspoon salt, and ½ teaspoon pepper, then whisk until well blended. Cover and refrigerate until ready to use.

Lightly flour the counter. Mist a 9-inch-round by 2-inch-deep tart pan with a removable bottom or a deep-dish glass pie plate with cooking spray. Unwrap the pastry disk and set it on the floured surface.

If the dough is too firm to roll, let it stand for 10 to 20 minutes. Dust the surface of the dough with flour and, using a rolling pin, roll it to a 14-inch round about ⅛ inch thick, rotating often and dusting with flour as needed to prevent sticking.

Fold the dough round in half, then in half again, forming a wedge. Transfer the dough to the tart pan or pie plate, positioning the tip of the wedge at the center.

Unfold the dough, then carefully ease it into the corners and up the sides of the pan or plate, allowing the excess to extend past the edge.

If using a tart pan, roll the rolling pin across the top of the pan to trim off the excess dough, then set the pan on a large plate (so the pan is easier to handle); if using a pie plate, fold and crimp the edges of the dough.

Refrigerate uncovered for at least 1 hour or cover and refrigerate up to 1 day. About 1 hour before baking heat the oven to 450°F with a baking steel or stone on the middle rack.

Stir the bacon and tarragon into the cooled mushroom mixture. Distribute the mixture evenly in the chilled pastry, then top with the cheese. Set the tart pan or pie plate on a rimmed baking sheet.

Slowly pour in the egg-cream mixture, allowing it to seep in; the pan will be very full. Carefully slide the baking sheet onto the hot baking steel, then immediately reduce the oven to 350°F.

Bake until the quiche jiggles slightly at only the center, 55 to 65 minutes (the center should reach 165°F to 170°F); lay a sheet of foil over the quiche if the surface browns too much.

Transfer the quiche from the baking sheet directly to a wire rack and cool until barely warm to the touch, about 2 hours. If you’ve used a tart pan, carefully remove the outer ring, then return the quiche (still on the pan bottom) to the rack. Cool completely.