Step 1

Preheat the oven to 200°C, gas mark 6. Move an oven shelf to the top 1/3 of the oven. Add 3 tbsp of the sugar and all but 2 of the halved plums to a mixing bowl. Combine well and leave to macerate for 10 minutes.

Step 2

Chop the remaining plums into 1-2cm dice. Transfer to a small pan and add the remaining 2 tbsp sugar, 1 sprig tarragon and 4 tbsp water. Bring to a rapid simmer and cook for 3 minutes, mushing the fruit with a fork. Take off the heat and drain the syrupy liquid through a fine sieve into a cup. Transfer the tarragon sprig to the syrup. Leave to steep. Combine the biscuit crumbs and melted butter to create a loose crumble, then set aside.

Step 3

Grease a large baking tray with a little oil, then line it with baking parchment. Unroll the pastry and set it on the parchment. Using the back of a knife, score a 2cm margin, without cutting through, around the edge.

Step 4

Sprinkle and spread the almonds evenly on the middle of the pastry (avoid scattering into the margin). With the tart in portrait aspect, arrange the plum halves, cut-side down, in 3 neat lines over the almonds. Using a flexible spatula, scrape any residual sugary juices from the pan over the fruit. Slide 6 of the tarragon sprigs underneath the plums. The tarragon will impart some of its flavour into the fruit juices as they cook out, and the almonds will prevent the base from becoming soggy.

Step 5

Carefully move the tart, on its tray, to the hot oven. Bake for 20-25 minutes, then remove, closing the door to retain the heat. Sprinkle the crumble over the plums and return to the oven for 5 minutes, until the pastry is golden and well-puffed and the plums soft and still juicy.

Step 6

Pick and finely chop the leaves from the final tarragon sprig. Using a pastry brush, paint the plum and tarragon syrup over the raised edge of the tart, pouring any excess over the plums. Sprinkle with the chopped tarragon and serve with the custard.