Preheat the oven to 190°C (375°F/Gas 5).
Pour the stock into a saucepan and add the quinoa, bring to the boil, then reduce the heat and simmer for 12 -14 minutes. Remove from the heat and leave to cool.
Fluff up the quinoa with a fork to separate the grains and stir through the onions and peppers, beetroot, sundried tomato, herbs and pine nuts and season with the extra virgin olive oil and balsamic vinegar.
Roll out the pastry on a lightly floured surface, cut out 6 x 15cm squares. Brush the portobello mushroom with mustard and Place into the centre of each pastry square then add the beetroot and quinoa mixture.
Brush the pastry edges with egg wash. Fold over to encase the mushroom and quinoa filling and seal the pastry.
Place in a preheated oven for 20–22 minutes until golden and crisp.
Serve immediately with a mushroom sauce made of cow milk or oat or nut milk!