Combine cooked wheatberries, milk, butter and zest of 1 lemon in a saucepan over medium heat. Cook, stirring often, until creamy, about 10 minutes. Remove from heat and let cool.
Preheat the oven to 200 C / Gas 6. Butter a a large 28cm (11 inch) cake tin and line with one of the pastry discs.
Combine ricotta cheese, sugar, eggs, egg yolks, 1 tablespoon orange blossom water, vanilla extract and cinnamon in the bowl of a food processor; mix until smooth. Mix in remaining lemon zest and mixed peel. Fold in wheatberry mixture and pour over the pastry base.
Slice the remaining pastry disc into 2cm wide strips. Place on top of the tart in a lattice pattern and brush with beaten egg.
Bake in the preheated oven until filling is cooked through and top is lightly browned, about 1 hour. Remove from oven and place on a cooling rack. Cool for about 30 minutes.
Remove tart carefully from the tin and place on a serving plate. Dust with icing sugar.
The special cooked wheatberries for this pie, called Grano cotto per pastiera (Italian cooked wheatberries) can be found in Italian grocery stores and online.