Heat 1 tbs oil in a large Sauté pan and fry the onions for 15 minutes, stirring regularly until browned. Ensure onions do not catch, as this will cause bitterness.
Remove onions from pan and set aside in a bowl to cool. Wash the spinach, then wilt in pan for 1-2 minutes. Squeeze any excess water out of the spinach then set aside in a bowl to cool.
Add remaining oil to pan and fry the mushrooms, whole for 4-5 minutes on each side, remove from the pan and allow to cool.
Pre-heat oven to 200°C, 400°F Gas Mark 6, unroll pastry, leaving on its grease proof paper.
Spread half of the onions lengthways over the centre third of the pastry leaving a gap of approximately 2 cm either end. Top with half of the spinach. Brush the mushrooms with the mustard and season well with salt and pepper. Place mushrooms atop the spinach, sprinkle with thyme and top with the remaining spinach and onions.
Carefully roll the pastry around the filling using the grease-proof paper and press the edges together with a little water to help seal then invert onto a grease proof baking tray so that the seal is facing down.
Score the top lightly in a diamond pattern and brush all over with the egg wash. Bake in the oven for 40 minutes, or until pastry is golden then serve immediately.