850g/1lb 14oz large onions
3 tbsp olive oil, plus extra for greasing
320g pack ready-rolled all-butter puff pastry
1 egg, beaten
2 tbsp Dijon mustard
25g/1oz fine breadcrumbs
50g/2oz grated Parmesan (or vegetarian alternative)
1/2 tsp thyme leaves
Finely slice the onions, then put them in a large, lidded pan with the oil and cook over a low heat for 30 mins, stirring from time to time.
Remove the lid and turn up the heat, cooking the onions until they brown and caramelise, stirring often.
Heat oven to 180C/160C fan/gas 4. Unroll the pastry and place on a greased baking sheet. Using a sharp knife, cut a 2cm border around the edge of the pastry, going almost but not quite through the pastry. Glaze the edge of the pastry with beaten egg.
Spread the mustard over the base of the tart and pile on the onions, spreading them out to meet the edge. Mix the breadcrumbs, Parmesan and thyme leaves together, and sprinkle this topping evenly over the onions. Bake the tart for 18-20 mins or until the pastry is golden brown.