Preheat the oven to 180°C. Line a baking sheet with baking paper.
1. Onions Heat the butter and oil in a pan and fry the onions over low heat until soft and translucent. Add the garlic and herbs and stir-fry until caramelised. Season with salt.
2. Pie Lightly roll out the pastry on a lightly floured surface. Spread the mustard on the pastry, followed by the onion mixture. Season with salt and freshly ground pepper.