Make the pie:

Preheat your oven to 375°F (190°C)

Arrange puff pastry as you like into your mold and put it back into your fridge, you want to use it very cold

If you’re going to make a lattice, make it now and put it back into the fridge until ready to use

Clean and peel the rhubarb (you don’t need to peel it completely, a little skin gives more flavor and you won’t feel it once cooked)

Cut the rhubarb in three lengthwise and in small chunks crosswise,

Take out puff pastry, with a fork poke holes, without completely piercing it, all over puff pastry, sprinkle it with the flour and about 2 to 3 tbsp. of sugar

Fill the mold with the rhubarb chunks, you can fill the mold to the brim, specially if you plan to cover the tart with a lattice, and also because the rhubarb will shrink

Make the quick custard:

Into a small bowl, add the egg yolk, sugar and cinnamon, whisk together, add the heavy cream (check that you have enough custard to cover your mold, add cream if necessary), whisk until all ingredients are incorporated, taste and adjust sweetness to your taste

Pour the custard evenly on top of the rhubarb

Cover the pie with the lattice, roll a rolling pin on the edges of mold to seal and cut the lattice

Sprinkle the lattice evenly with icing sugar

Bake the pie:

Into the oven for about 25 to 35 minutes, or until puff pastry is golden

Let cool into the mold on a rack