All-butter frozen puff pastry such as Dafour is a treat, but oil-based frozen puff pastry, such as Peppridge Farms, readily available in most grocery stores, is just fine. Inspired by Izy Hosack's frame tart! If you're making this vegan, leave out the labneh sub more apricot jam for the egg wash.
Thaw the puff pastry at room temperature for 40 minutes. Place the oven rack in the center of the oven, and heat to 400 degrees, and line a sheet pan with parchment.
If using Pepperidge Farms, unfold one of the puff pastrysheets on the parchment-lined sheet pan and prick all over with a fork. If using Dafour, unfold the pastry sheet and cut it into about a 9-inch square. Cut 4 1½-inch wide strips from the other sheet or remaining pastry. These will form a perimeter second layer on the square piece.
Brush the perimeter of the pastry square lightly with beaten egg to create a glue for the strips. Lay the strips around the perimeter with the egg wash, cutting the strips to fit so that they don’t overlap at the corners. Brush the tops of the strips with egg wash.
Sprinkle half of the pistachios evenly over the center of the pastry square. Spoon dollops of labneh in several spots over the pistachios. Spread the labneh using an offset knife, without lifting it off the pistachios as you go (it’ll be easier to spread the less the knife is lifted).
Measure the length of the internal square of the
pastry, and cut several pieces of rhubarb to this length. Lay the rhubarb pieces
snugly next to each other to fill the pastry.
In a small bowl, rub the sugar with and lemon zest with your
fingers to release the lemon flavor. Add the rose water and mix to combine.
Sprinkle the sugar mixture evenly over the rhubarb.
Bake for about 20 minutes, or until the pastry is deep golden brown and the rhubarb is soft. Cool the tart for 10 minutes. Warm the apricot jam for a few seconds in the microwave then brush the tops of the rhubarb with the jam.
Sprinkle the pistachios in rows between the rhubarb slices; this is easiest using your fingers to control where the pistachio lands.
Slice and serve warm, or at room temperature.