3: Preheat the oven to 390°F (200°C). Put 20 g butter in a small ramekin and put in the oven while it is warming up. This is a good tip for using the oven heat if you are going to need melted butter later .
6: This is not essential, but you can glaze the edge of the pastry (1/2 inch or 1 cm) using a brush.
7: Spread 50 g ground hazelnuts over the puff pastry, leaving the edge (about 1/2 inch or 1 cm) clear.
8: Arrange the plums in rings, standing them on edge. A tip to get started: Put something with a bit of weight to it (a plastic block here) and lean the first plum against it, taking it away to complete the ring.
9: Complete the filling. As with any tart, it needs plenty of fruit.
10: Take the melted butter out of the oven. Remember to be careful as the dish will be hot. Brush the tops of the fruit generously with butter.
11: Sprinkle brown sugar generously over.
12: Bake for 25 minutes.
13: After this time, take the tart out of the oven. Lift it on the cooking parchment and slide it onto oven-proof wire rack. Put the tart back in the oven for a further 10 to 15 minutes (watch for browning). This is the secret of a really crisp tart.
If don't have ground hazelnuts, use ground almonds instead. You can vary this idea with different fruits, according to the season: mirabelles or apricots for instance. If using commercial ready-rolled pastry, this will take a good 2 pounds (1kg) of plums and 3 oz (80 g of ground hazelnuts.