• Preheat the oven to 425°F (220°C) and line a large baking sheet with parchment paper.

Cook the filling:

• Heat the olive oil or vegan butter in a large skillet over medium-high heat. Add the onion and cook for 3–4 minutes, stirring occasionally, until translucent.
• Stir in the mushrooms, red wine, vegetable stock powder, salt, and pepper. Cook uncovered for about 10 minutes, stirring occasionally, until the mixture is softened and most of the liquid has evaporated.

Prepare the pastry:

• Place the two puff pastry sheets on the prepared baking sheet, overlapping them slightly in the center to form one large rectangle. Press the seam together using the back of a fork to seal.

Assemble the galette:

• Spoon the mushroom mixture into the center of the pastry. Spread it out evenly, leaving about 1½ to 2 inches around the edges. Fold the pastry edges up and over the filling to create a rustic border, pleating as needed.
• Bake for 25 minutes, or until the pastry is golden brown and crisp.

Serve:

• Let cool slightly. Top with crumbled vegan feta, a handful of arugula, a drizzle of balsamic glaze, and a pinch of sea salt flakes if desired. Slice and serve warm.