Blood orange upside-down tart with pomegranate rubies

Preheat the oven to 220°C.
Peel and slice 1 or 2 blood oranges, enough to cover the bottom of a round, shallow buttered baking tray or ovenproof pan. Sprinkle with enough brown sugar and top with a few dollops of butter. Cut a circle from ice-cold ready-made French puff pastry, big enough to cover the oranges and the inside of the pan or baking tray.
Place in the oven and bake for 20 minutes, until the crust is golden brown and cooked through.
Remove, carefully turn onto a plate, score slightly with a gas burner to caramelise the sugar, then sprinkle with pomegranate rubies.
Serve hot or cold with yoghurt, crème fraîche or cream on the side.