MAKE THE FILLING. Wash and dry the mushrooms with a paper towel. Chop them finely and set aside. (You can use a food processor to save time).
Into a large, non-stick skillet, add the olive oil and onion. Cook over medium-high heat for 5 minutes, or until the onion is soft and golden.
Add the mushrooms to the onion and cook for about 10 minutes, until the mushrooms are fully cooked through and starting to brown.
At this point, add the garlic, oregano, miso, soy sauce, salt, pepper, and spinach. Cook until the spinach is fully wilted (about 3-5 minutes). Divide the filling into four equal parts, set aside, and let it cool.
FILL THE TWO PUFF PASTRIES. Carefully unroll the puff pastry onto a silicone or parchment-lined baking sheet. Place a quarter of the mushroom filling in a mound along the center (see photo). Then arrange the pumpkin slices in a single layer on top. Repeat the mushroom and pumpkin layer. Working quickly, close the pastry. Do the same to the second pastry sheet.
SCORE THE PASTRIES. Using a razor blade or a sharp knife, score the pastries (use cross-hatch, herringbone, leafy vine, or just simple diagonal cuts).
ADD "EGG" WASH. Brush the pastries with vegan "egg" wash (1 tbsp. olive oil + 1 tbsp. of plant butter mixed together).
BAKE. Bake at 400°F for 30 minutes, checking at 20 mins. Let the pastry bake until it is a really deep golden color – to ensure it’s done and flaky all the way through. You may need to add 5 more minutes, depending on your oven.