Rhubarb Frangipane Slab Pie

PIE DOUGH

1. Place flour, sugar and salt into a large bowl. Cut butter into chunks, and add to the flour. Toss lightly to coat. Working quickly, using a pastry blender or  your fingers,  cut the butter into the flour mixture until there are only pea-sized chunks left. You want a few lumps of butter remaining to keep the pastry nice and tender.
2. Combine ice, water and cider vinegar in a bowl. Sprinkle a few tablespoons of the ice water into the flour and butter mixture, and using a stiff spatula or your hands, mix in well. Continue adding water a tablespoon at a time ( I normally need about 10-12 tbsp) until you have a dough that holds together well, but is not too wet. Squeeze together with your fingertips to make a homogenous dough. Shape into two rectangles, one slightly larger than the other, and wrap in plastic wrap. Rest in the fridge for at least two hours, or preferably overnight. 

ROASTED RHUBARB

1. Preheat the oven to 350˚f / 180˚c. Arrange the rhubarb in a 9" x 13 baking sheet. Split the vanilla bean lengthwise using a sharp knife, and nestle in the rhubarb. Combine the sugar, water and salt in a small bowl until the sugar has dissolved, and pour over the rhubarb. 
2. Bake for 20-25 minutes, until the rhubarb is just tender. Remove from the oven and cool. Remove the rhubarb from the syrup, and place on a paper towel to drain. Reserve the syrup - it is perfect for cocktails or soda!

FRANGIPANE FILLING

1. In the bowl of a stand mixer fitted with the paddle attachment, combine the butter and sugar until just combined. Add the almond meal, egg, vanilla, and salt, and mix until incorporated. Set aside until required. 

ASSEMBLY AND BAKING

1. Roll out the smaller rectangle of dough until it is approximately ⅛" (3mm) in thickness. Line an 11" x 8" removable bottom tart tin with the dough, trimming any overhang flush with the edge of the tin. Keep any scraps and re-roll to use for decoration - I usually use these pieces for pastry stamps
2. Using an offset spatula, spread the frangipane evenly in the bottom of the lined pie tin. Top with the roasted rhubarb, pressing down gently on the pieces. 
3. Roll out the second rectangle of dough, and cut strips to be used for a lattice. Lattice however you like - for this one I did a range of different width strips, cut with both a straight cutter and a fluted cutter, and incorporated some braids into the lattice too. Keep any scraps to use for stamping if desired. 
4. Place the finished pie in the fridge for at least 30 minutes. While the pie is resting, preheat the oven to 425˚f/ 220˚c. Place a baking tray on the bottom rack of the oven. 
5. Brush the pie with egg wash, and sprinkle liberally with raw sugar. Bake at 425˚f / 220˚c for 20 minutes, or until the pastry is set and beginning to go golden. Reduce the temperature 375˚f / 190˚c, and bake until the pastry is deeply golden and the filling is bubbling. 
6. Remove from the oven and cool on a wire rack. Serve warm or at room temperature.