Steam-Baked Mushroom Wellington with Port Sauce

Combine the flour and salt in a food processor. Pulse to combine. Add about three-quarters of the butter and pulse until the butter forms pea-sized pieces. Add the remaining butter and pulse a few more times to combine. Add the water and continue to pulse until the dough comes together. Transfer to a lightly-floured counter and pat into a rectangle. Use a rolling pin to roll into a 12- by 18-inch rectangle. With one short side facing you, fold the top edge two-thirds of the way down the rectangle, as if you were folding a letter. Fold the bottom edge up to meet the top of the dough. Wrap in plastic and refrigerate for at least two hours or up to overnight.

Use a paring knife to remove the thin skin from the top of the portobello mushroom caps by peeling from the edge of the cap towards the center. Transfer to a large bowl. Add the mixed mushrooms, olive oil, shallots, garlic, 2 tablespoons balsamic vinegar, the soy sauce, rosemary, and thyme. Toss well to coat and season with salt and pepper. Transfer the portobellos to a sheet pan or vacuum-seal bag. Place the remaining mushrooms in a separate sheet pan or vacuum-seal bag. If using bags, seal.

Transfer to the oven and cook for 1 hour. Let cool to room temperature and then refrigerate until cold (see note).

While the mushrooms cook, melt the butter in a large skillet over low heat. Add the onions, brown sugar, and remaining tablespoon balsamic vinegar. Cook, stirring occasionally, until caramelized, about 1 hour. Season to taste with salt and pepper. Remove from the heat and let cool to room temperature. Transfer to the refrigerator and chill until cold.

If you’d like, you can turn off the oven after removing the mushrooms and then return to this stage after assembling the Wellington.

Thoroughly drain the mushrooms through a fine-mesh strainer set over a bowl. Set the jus aside for the port sauce. Transfer the mixed mushroom mixture to the food processor. Pulse until finely chopped. Leave the portobello mushrooms whole.

On a lightly floured counter, roll the chilled pastry dough into a 12- by 15-inch rectangle. Transfer to a parchment- or silicone mat-lined baking sheet. With one long side of pastry facing you, spread the onions into a 4- by 10-inch rectangle in the center of the pastry, leaving a 1 inch border on the top and bottom and about 5 1/2 inches of bare pastry on both sides. Layer two-thirds of the chopped mushroom mixture onto the onions. Place the portobello mushroom caps in a row on top of the chopped mushroom mixture, and then add the rest of the chopped mushrooms on top. Think of this process like shaping and stacking a freeform meatloaf. You want there to be some structure to it.

Brush the edges of the pastry with some of the egg wash. Fold the long sides of bare pastry over the mushrooms, then fold and crimp the ends to seal. If you feel you have too mush pastry, feel free to cut it down to size. You can use scraps to cut decorations for the top of the Wellington. Slice decorative vent holes on the top of the Wellington, and then brush all over with egg wash.

Transfer to the oven and bake until golden brown, about 45 minutes.

While the Wellington bakes, heat the olive oil in a medium saucepan over medium heat. When the oil is shimmering, add the shallots and cook, stirring, until softened. Stir in the reserved mushroom jus and thyme. Bring to a simmer and stir in the port. Continue to simmer until the sauce has thickened to a medium-bodied texture. Strain through a fine-mesh strainer into a medium bowl and whisk in the butter.

Let the Wellington rest for 10 to 15 minutes. Slice and serve with the port sauce.