See attached video on how to assemble.
To assemble the gâteau, you will need a 9" to 10" -inch metal baking ring.
Remove one-third of the cold dough from the refrigerator. Generously flour a large piece of parchment paper and place the dough in the center.
Roll the dough into a rough circle, until it's about 1/4" -inch thick, and a few inches larger than the diameter of the ring. Using the ring like a cookie cutter, press it into the dough. Remove the excess dough from the outside and separate it into two pieces. Store one piece in the refrigerator while you assemble the gâteau.
Roll out the second piece of dough into a rope long enough to line the inside of the ring. Using floured fingers, press the dough against the sides of the ring, and seal the edge and base together.
Once the wall is about 1"- to 1 1/2" -inches high, use a pairing knife to even out the top. Peel away the excess dough and reserve it for the top crust. Gently even out the edge and carefully transfer the shell to a piece of aluminum foil. Wrap the foil up the sides of the ring to make sure it’s nice and snug.
Transfer the shell again to a baking tray and remove the pastry cream from the refrigerator. By this time it will be cold and set, so you'll need to whisk it with a sturdy wire whisk to smooth it out. Pour the pastry cream into the shell and smooth out the surface.
Next, roll out the reserved piece of dough, making it large enough to cover the top. The top layer will be quite thin, so be careful when transferring it over top the gâteau. Trim off any excess dough and press the top and edge together to seal.
Whisk a whole egg very well and gently brush the top of the gâteau. Sprinkle with a bit of sugar and place into a COLD oven. Set the oven to 325° degrees Fahrenheit and bake for approximately 1 hour, or until golden brown.
Once the gâteau has finished baking, remove from the oven and let cool for approximately 15 to 20 minutes. Run a very sharp knife between the gâteau and the ring to release. Remove the ring and let cool completely before serving. Dust with icing sugar, if desired.