For candied orange, place orange slices in a large saucepan and cover with cold water. Bring to the boil, drain and repeat 7 times. Combine sugar, corn syrup and 80ml (1/3 cup) water in a large saucepan, stir over medium heat until dissolved and simmering. Add orange slices, bring to the boil, simmer over low heat until orange is translucent and syrup is thick (1-1½ hours). Remove from syrup and drain on wire racks in a single layer until cool. Reserve ¼ cup syrup, and add grappa, for brushing.
For almond filling, bring milk and orange rind just to the boil in a small saucepan over medium heat. Whisk egg yolks and half the sugar in a bowl until thick, add flour and whisk to combine. Pour over milk mixture, whisk to combine, return to pan and whisk over medium heat until thick (2-3 minutes). Cool completely, remove rind. Beat butter and remaining sugar using an electric mixer until pale, add grappa, almond meal and cooled custard and beat to combine.
Preheat oven to 180C. Roll out pastry to a 30cm round between 2 sheets of baking paper. Refrigerate for 30 minutes. Spread almond filling over pastry, leaving a 4cm border, top with orange slices and fold over pastry edges, pleating as you go. Brush pastry with eggwash, then brush oranges with reserved syrup. Bake until golden (35-40 minutes). Cool on tray for 10 minutes, transfer to a cake rack and cool. Serve drizzled with remaining syrup.