Cook the farro in water for 40 minutes until just tender. Leave to cool in its liquid.
Strain farro and place into a pot with milk, lemons zest, and vanilla. Bring to just under a boil then reduce heat and gently cook until all of the liquid is absorbed.
Stir in the ricotta until creamy, then add the sugar and stir incorporated. The farro will continue to absorb the liquid off the heat. When the farro is cool stir in the candied peel. (This can be done one day in advance.)
Place cream, sugar, and salt into a heavy bottomed pot. Bring to a boil then remove from heat and allow to infuse for 5 minutes. While the cream mixture is still hot, temper in yolks.
Pour the custard into the ricotta filling and pour into prepared tart shell. Place the tart shell into the oven and bake until just set about 20-25 minutes.
Remove from heat and allow to cool. Slice and dust with icing sugar.