Method
1. Place mixed mushrooms (in batches) and the slices of bread in a food processor, pulse until roughly chopped. Transfer to a large bowl.
2. Trim the portobello mushrooms by slicing off the stem to make a flat surface. Place the excess stems into the bowl of chopped mushrooms.
3. Brush both sides of the portobello mushrooms with olive oil. Season.
4. In a fry pan over medium high heat, drizzle some olive oil then cook mushrooms 4-5 minutes on each side until caramelised. Transfer to a wire rack, grill side down to cool and remove excess moisture.
5. Return the frypan back onto the heat, drizzle olive oil and cook the chopped mushrooms (in batches) until golden brown. Transfer to a bowl to cook then add the walnuts (if using), thyme and parmesan.
6. Drizzle a little more olive oil on the frypan, add the onions and cook stirring occasionally until softened and caramelised, 5-6 minutes. Add the balsamic vinegar, season with salt and pepper and cook for a further 5 minutes until all the liquid has evaporated. Take off heat and set aside to cool.
7. Preheat the oven 200°C.
8. Place the puff pastry on a piece of greaseproof paper on a baking tray. Arrange half the cooked mushroom mixture in a strip in the centre of the pastry. Layer the onion jam, then arrange the portobello mushrooms on top in a single layer. Spoon the remaining mushroom mixture covering the portobellos.
9. Lift one side of the pastry over the filling, brush the tops with the beaten egg then lift the other side of the pastry, folding it over the egg washed pastry. Gently press down to seal the two pieces of pastry together.
10. Brush the insides of either side of the pastry, press to seal then tuck both sides underneath.
11. Brush the top of the puff pastry with the remaining beaten egg. Decorate the tops of the pastry as you like. Bake in the oven for 45-50 minutes or until the pastry is golden.
12. Remove from the oven, and let it cool for 10 minutes before cutting.