Preheat oven to 275°F. Line a small sheet tray or plate with paper towels and set aside.

Trim carrots, parsnips and leeks to 8 inches long, removing the dark green tops from the leeks.

In a 9×13 pan, add all ingredients. Cook for about 1 ½ hours and remove from oven. Using a toothpick, check to see if vegetables are tender, but still have light resistance when toothpick is inserted. Remove any tender vegetables and place on prepared sheet tray. If any vegetables need to continue cooking, turn to coat in oil again and cook for an additional 30 minutes, or until tender.

Once cooled, transfer remainder of vegetables from oil onto prepared sheet tray and set aside. Strain and reserve oil for other use.

Tear cleaned mushrooms into 1 to 2 inch pieces. Add ¼ of the mushrooms to the bowl of a food processor. Pulse and process about 10 times, or until mushrooms are about ¼ inch wide. Transfer to a large mixing bowl and repeat until all mushrooms are processed.

In a 12-inch skillet over medium low heat, add oil and butter. Once butter has melted, add shallots and a generous pinch of salt. Cook for 2 minutes, or until shallots begin to soften and turn translucent. Add mushrooms, thyme and garlic and stir to combine. Cook for about 15 minutes, stirring occasionally, or until mushrooms have released most of their excess water and the mixture reduces significantly. Add Marsala wine and Dijon mustard, stir to combine and cook for 2 minutes, or until Marsala has cooked off and the mixture is mostly dry. Taste season with salt and pepper.

Transfer to a resealable container and refrigerate until completely chilled.

Once duxelles and confit vegetables are chilled, lay out a 14×18 sheet of plastic wrap (combining two sheets, if needed) on a clean work surface. Add all duxelles onto plastic wrap and using an offset spatula or small spatula, spread until mushrooms measure 11×11, and about ¼ inch thick.

Arrange carrots in one layer in the middle of the mixture so that one thick end is beside a thin end, and they are nestled tightly together. Next, place leeks centered on top of the carrot layer, then carefully sandwich parsnips on top of the leeks, alternating thick and thin ends to fill in as many gaps and crevices in the leeks and carrots below. The final shape should be as cylindrical as possible, and you may not use all 4 parsnips depending on the size.

Working quickly, take the far end of the plastic wrap and pull it towards you, covering the vegetables in duxelles. Continue to pull the plastic wrap towards you, keeping tension on the plastic end in front of you, to fully encase vegetables in duxelles (similar to rolling a jellyroll). Fold the ends of the plastic wrap in, like closing a burrito, and close as tightly as possible. Pull out a second sheet of plastic wrap of the same size and place wrapped log on the lower third of the plastic. Fold the long side closest to you over the log and then roll it away from you to fully encase it in a second layer of plastic. Grip the two ends of the plastic wrap, close to the base of the vegetable log. Then on your work surface, roll the log away from you while still holding the ends of the plastic wrap, sealing the log and removing any excess air from the vegetable mixture.

Transfer to refrigerator and chill overnight, or at least 10 hours.

When ready to fully assemble Wellington, preheat oven to 425°F.

Place a piece of parchment paper on a clean work surface and lightly flour the paper. Add 1 sheet puff pastry and gently unroll it, lightly pressing the seams together. Working quickly, roll pastry out to 13×13 inches. Transfer parchment and dough to a sheet tray and place in the refrigerator while you roll out the second piece to the same size.

Once the second sheet has been rolled out, set aside while you finish preparing the first sheet. Remove the first sheet from the refrigerator and brush the surface with egg wash. Top with second sheet and brush with Dijon mustard.

Remove vegetable log from refrigerator and carefully unwrap it and quickly transfer it to the bottom third of the puff pastry. Pull the long edge closest to you over top of the vegetables, brush the edge with egg wash, and then carefully continue to roll the vegetable log away from you, to fully enclose it in pastry. Then lightly brush the ends with egg wash and crimp them underneath the Wellington, similar to wrapping a gift.

Using a paring knife, carefully cut vents into the top of the Wellington, being sure not to cut fully through the pastry. You can cut a crosshatch pattern, or several small V shaped cuts across the top. Brush the entire surface with egg wash.

Bake for 40-45 minutes or until deeply golden brown on the outside. Remove from the oven, allow to cool and make green peppercorn sauce.

In a small skillet over medium heat, add butter. Once melted, add shallot and a generous pinch of salt and stir to combine. Cook for 2 minutes, or until soft and translucent. Then add peppercorns and cook for 30-45 seconds, or until fragrant. Sprinkle flour over top and stir to combine. Cook for 1-2 minutes, or until the flour turns blonde.

Add stock and Dijon and whisk until mustard is fully incorporated. Bring to a boil, then reduce heat to a simmer and cook for 6-8 minutes, or until sauce has thickened and reduced to the texture of a thin gravy. Taste and season with salt. Cover with lid until ready to serve.

Allow to cool for 30-45 minutes, or until the Wellington is warm to the touch.

Slice and serve with Green Peppercorn Gravy.