- For the POM Dijonnaise: In a small bowl, stir together the mayonnaise, Dijon mustard, POM Juice, salt, and pepper. Store in the fridge until ready to use.
- For the Mushroom Wellington: Heat the oven to 400ºF. Line a baking sheet with parchment paper. Trim the stems of the portabella mushrooms and remove the gills using a spoon. Place the portabellas on the baking sheet and roast until tender, about 15 minutes, flipping halfway through. Remove from the oven. Allow to cool completely and discard any excess liquid.
- Add the cremini and button mushrooms to the bowl of a food processor fitted with the blade attachment. Pulse until roughly ground.
- Heat the olive oil and butter in a large sauté pan over medium-high heat. Add the shallots and garlic and cook until softened, about 2 to 3 minutes. Combine the ground cremini and button mushrooms with the oyster mushrooms, and cook until tender, about 6 to 7 minutes. Add the thyme, rosemary, salt, and pepper, and cook an additional minute.
- Deglaze the pan with the mushroom mixture using POM Juice and balsamic vinegar. Allow to reduce until almost dry, about 10 to 12 minutes. Remove from heat and let cool.
- On a lightly floured surface, roll out the puff pastry to an 11"x10" rectangle. Place half of the cooled mushroom mixture in the center of the puff pastry. Place the sliced portabella mushrooms, cap-side down, on top of the mixture. Spoon the remaining half of the mushroom mixture on top and gently press down to pack the mixture onto the mushroom caps.
- Brush the puff pastry with egg wash. Fold both sides of the puff pastry over the mushroom mixture like a letter and press to seal. Fold the two short ends of the Wellington like you are wrapping a present and press to seal. Place seam-side down on the baking sheet.
- Using a paring knife, make three long vertical lines down the center, then short diagonal lines off of the center lines—do not cut through the pastry, as this is just for design. Cut a few small holes in the Wellington ends to act as vents. Brush the egg wash over the top of the puff pastry.
- Bake until golden brown and cooked through, about 40 to 45 minutes, rotating the pan halfway through. Allow to rest for at least 10 minutes before slicing and serving with the POM Dijonnaise. Garnish the slices with POM Arils and parsley.
Tip: To make this dish vegan, swap the mayonnaise for vegan mayonnaise, the puff pastry for vegan puff pastry, and the butter for vegan cultured butter, and use aquafaba in place of the beaten egg.