Preheat the oven to 180°C (160°C fan oven) Gas Mark 4. Grease a 20cm x 30cm tin with a little of the butter. Press in a sheet of filo, leaving the excess overhanging, and brush with more melted butter. Add a second sheet, crossing the pastry in the other direction. Brush again with butter, and repeat using 3 more sheets of the pastry, criss-crossing the direction each time.
Mix together the ground nuts, mincemeat, hazelnuts, walnuts, cinnamon, orange zest and sugar. Add the mixture to the tin and flatten it evenly to firm up. Fold over the overlapping pastry, back over the filling. Brush with butter and top with the final two sheets, brushing butter between layers and folds.
Brush the top with more melted butter, and then using a sharp knife cut diagonal or straight lines through the top layers of pastry to make diamond or square shapes. Bake for 25-30 minutes until golden brown.
When the baklava comes out of the oven, mix together the orange juice and honey and brush all over the top. Allow to cool completely before following the cuts you made before baking, to completely cut into pieces.