Store the leftover caramelised onion for 1 week in the fridge. For longer storage, pour into sterilised jars and seal. Store the unopened jars in the pantry for up to 6 months.
Easy to adapt and make it gluten-free by using gluten-free flaky pastry.
Make it vegan by using vegan flaky pastry, vegan cheese and plant-based milk for brushing, omit anchovies.