Place a large frying pan over low to medium-low heat. Add the 1/2 Tablespoon of olive oil followed by the onions and reduce heat to low. Season with salt and pepper and cook, stirring occasionally, for 15 to 20 minutes, until the onions are golden brown. Keep an eye on the onions to make sure they don't catch.
Remove the onions from the pan, and return the pan to the heat. Add the baby spinach and cook until wilted. Remove the baby spinach from the pan and leave to cool.
Increase the heat to medium-high and return the pan to the heat. Add the remaining olive oil and place the mushrooms in the pan, top-side down. Cook until lightly golden (about 5 minutes) before flipping and cooking for another 5 minutes or until golden. Remove from the heat and drain on a paper towel top-side up. They will release a lot of liquid as they cool. Transfer the onions, spinach and mushrooms to the refrigerator and cool completely.
Preheat the oven to 390 degrees. Place a sheet of baking paper on the baking tray and then place the puff pastry sheet on top. Spread half the caramelized onions over the middle third of the pastry, making sure to leave an extra 2cm (3/4 inch) border at the edge of the pastry. Top with half of the baby spinach. Spread the dijon mustard over the mushrooms and season well with salt and pepper. Place the mushrooms on top of the spinach. Top the mushrooms with thyme and the remaining baby spinach and onions.
Very carefully roll the pastry over the top of the mushroom mixture until you have a log. Press down to seal the edges. Roll the log over so that the seam is facing the bottom.
To make the vegan egg wash, whisk all of the ingredients together in a bowl. Very lightly coat with the vegan egg wash. Place the pastry in the freezer for 10 minutes before repeating with another layer of vegan egg wash and freezing the pastry for a further 10 minutes.
Place the pastry back on the baking sheet and place in the oven for 30 to 35 minutes or until golden and flaky.
The mushroom Wellington is best eaten as soon as it comes out of the oven as the pastry will start to soften when the mushrooms cool and release juices. I have added pre-baked sliced potatoes to this recipe. Roast then add to the layering in the puff pastry. Baking does not change. This offers a meatier vegan Wellington!
For the vegan egg wash, I have brushed "Just Eggs" over the top, and it makes a nice coat.
Wife and publicist of Dr. David Bernstein, Melissa has developed many clinical educational offerings online, published articles and co-authored several books over the years. Her love for cooking is put into action as she helps others live out the Power of 5 lifestyle!
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