Combine flour, salt, seeds and black pepper in a bowl. In a separate bowl whisk together egg, egg white, water and olive oil.
Add the wet ingredients to dry and stir together with a fork. Once mixed thoroughly turn the dough out to a lightly floured surface and knead until all the dough comes together. Add more flour if necessary. Place the dough in the refrigerator while making the filling; you can leave it to rest for up to two days in the fridge.
Preheat the oven to 190 degrees Celsius or 375 degrees F.
Slice the acorn squash into 1/2-inch thick semicircles and scoop out the fibres and the seeds. Place on a rimmed baking sheet. Slice the onions into 1/4-inch semicircles and add to the baking sheet as well. Smash garlic and toss it onto the baking sheet.
Add cleaned, sliced mushrooms onto baking sheet. Drizzle vegetables with olive oil and season with salt and pepper. Roast until vegetables are softened and browned, about 25 minutes.
Let cool then scrape the squash from their skins. Mix squash filling with onions, garlic and mushroom. And add cumin and nutmeg. Season to taste.
Divide the dough into 6 even balls and roll them out to 6 8x8-inch squares. Add about 1/4 cup filling into the centre of each pastry. Lightly brush the edges of each pastry with egg wash. Now bring two opposite corners of the dough towards the centre and pinch together. Life the two remaining corners up to the centre, matching the seams so they meet to form a raised ridge. Crimp the edges to seal. Brush with egg wash.
Repeat with remaining dough and filling. Place finished pockets onto a baking sheet lined with parchment paper. Bake for 25-30 minutes or until the tarts are golden brown and bubbling. Let cool for about 10 minutes before serving. Serve with your favourite chutney!