Line a baking tray with parchment (baking) paper.

Heat half the butter in a sauté pan. Brown the garlic in butter and add the mushrooms. Cook until the mushrooms are soft and the water has evaporated and set aside.

Heat the remaining butter and cook the onions till soft. Add the spinach and cook until all of the water has evaporated. Combine the spinach and mushrooms to form the filling and let cool. Season the mixture with salt and pepper.

On a lightly floured work surface, roll out the pastry dough into a 12x18-inch sheet. Cut the sheet lengthwise from the center to make two 6x18-inch strips. Lightly fold one sheet lengthwise and at a gap of 2-inch, make 1-inch cuts on the folded side. Place the other sheet on the prepared tray and spread the filling leaving 1-inch from the edges. Top the filling with cheese. Brush the edges with egg wash and cover the filling with the previous sheet. Press the edges with the back of a fork to seal the Jalousie. Cover the tray and refrigerate for 30 minutes.

Preheat the oven to 180°C/350°F. Brush the top of the Jalousie with egg wash and bake until the Jalousie is golden brown, about 30 minutes. Transfer the Jalousie to a wire rack and let cool for 15 minutes. Slice crosswise and serve immediately or warm.

Variation: Filling: Try dry sabzis like jeera-aaloo, sookhi-matar, paneer-bhurji, etc
Shape: Cut the puff sheet into twelve 4-inch squares. Brush the edges with egg wash and drop a tablespoon filling in the center. Fold over the square diagonally and seal the edges to form triangles. Refrigerate and bake as directed.