Heat the olive oil and butter in a wide saucepan over a medium heat and add the red onions, thyme sprigs, bay leaves, caster sugar and a pinch of salt. Stir to coat. Pour in just enough boiling water to cover, and bring to the boil. Simmer for 10-15 mins until the water has completely evaporated.

Continue to cook for 10-15 mins until the onions start to caramelise and become sticky. Stir in the plain flour and cook for 2 mins before splashing in the balsamic vinegar and red wine. Bubble and reduce until the alcohol has evaporated, then pour in the vegetable stock. Bring to the boil and cook for 8-10 mins until thickened and slightly reduced. Season to taste, then remove from the heat and cool completely. Transfer to an airtight container. Will keep chilled for up to four days or frozen for up to three months. Use this to make our roasted cauliflower cheese toad-in-the-hole with red onion gravy.