Bring wheat grains, milk, butter, and 1/2 of the lemon zest to a simmer in a saucepan over low heat. Simmer until creamy, about 10 minutes.
Preheat the oven to 400 degrees F (200 degrees C). Grease a 11-inch tart pan.
Combine ricotta cheese, sugar, eggs, egg yolks, orange flower water, vanilla extract, and cinnamon in a bowl; beat with an electric mixer until well combined. Mix in remaining lemon zest and candied orange peel. Fold in wheat mixture and mix filling until well combined.
Roll out 1 sheet puff pastry on a lightly floured surface so it will fit into the tart pan. Place pastry in the pan and spoon filling on top. Roll out the second puff pastry sheet and cut into 3/4-inch thick strips. Layer a lattice pattern on top of the filling and brush with beaten egg.
Bake tart in the preheated oven until filling is set and lattice is lightly browned, about 1 hour. Remove from the oven and allow to cool inside the tart pan, at least 1 hour.
Carefully unmold tart, transfer onto a cake platter. Dust with powdered sugar before serving.