Preheat the oven to 180°C/Gas Mark 4. Add both tins of onions to a roasting tin and roast in the oven for 45 minutes until browning and sticking to the edges. Remove and set aside to cool.
With the remaining onions still in the roasting tin, crumble over the stock cube. Sprinkle over the flour and cook on the hob, stirring for 1 minute. Add 1 ltr (13⁄4pt) boiling water and stir, to bring everything together. Cook over a low heat for 5 minutes, scraping the bottom and sides of the tin. Keep warm.
For the potatoes, drain the tin and carefully press each potato so that it is slightly squashed. Heat a roasting tray in the oven with a little oil and roast the potatoes until crispy, around 50 minutes.
Drain the mushrooms and add to the pan, followed by the lentils. Lower the heat and simmer for 3 minutes or until the mixture has thickened and the liquid has evaporated. Take off the heat and add the lemon zest and parsley, then combine and set aside to cool.
Sauté the spinach in a little oil, then crumble in a vegetable stock cube. Cook for 2 minutes, season, then set aside to cool.
To assemble, roll out the pastry sheet, leaving it on its paper. Brush the mustard across the pastry, leaving 2.5cm (1in) to the edge of the pastry.
Spread the reserved onions, then the spinach and finally the mushroom and lentil mixture onto the pastry. Roll the pastry by lifting the paper, seal the edges, then fold the ends underneath. Brush with oil and return to the fridge for at least 30 minutes.
Remove the paper and place on a baking tray. Brush again with oil and roast in the oven for 45 minutes or until golden brown.