Preheat the oven to 400ºF and place a rack in the center of the oven.

Toss the rhubarb with the sugar and orange zest in a large bowl and set aside.

In the bowl of a food processor, pulse the flour, sugar and salt together.

Cut the butter into small pieces, then add to the food processor.

Pulse at 1-second intervals until butter is the size of peas—should be about 10 quick pulses.

Add the ice water and pulse again about 10 times until the mixture is crumbly but holds together when pinched.

Lay two clean tea towels on a work surface. Dump half of the crumbly dough mixture into the center of each.

Grab the four corners of the towel together and twist to create a beggar’s purse, pressing the dough into a round.

Use your hands to pack and flatten the round. Store one of the rounds in the freezer for a future use.

On a lightly floured work surface, roll out the dough into a 12- or 13-inch round.

Transfer dough to a parchment- or Silpat-lined sheetpan.

If you have space in the fridge, chill the pastry while you make the frangipane.

Combine almond flour, sugar, salt, butter, egg, and vanilla in the uncleaned bowl of the food processor. Purée until smooth.

Spoon the frangipane into the center of the rolled out dough leaving a 1- to 2-inch border.

Heap the rhubarb and all of the juices into the center of the frangipane and spread out to cover.

Fold the exposed edge of dough towards the center to make a rustic enclosure.

Brush the edge of the dough with melted butter. Drizzle the remainder over the exposed rhubarb.

Sprinkle the sugar evenly over the top.

Bake for 35 minutes or until golden.

Remove pan from the oven and let rest on cooling rack for 5 to 10 minutes.

Cut into wedges. Serve on its own or with vanilla ice cream or whipped cream.