To prepare pastry, combine flour, sugar and salt. Cut in butter until the texture of coarse meal. Add egg yolk and milk and blend just until dough comes together. Wrap in plastic wrap and chill while preparing fruit.
For fruit, toss walnut pieces, brown sugar, flour, butter and spices. Pull out ¼ cup of mixture and set aside. Add plums to remaining nut mixture and toss.
On a lightly floured surface, roll out pastry to just shy of ¼ -inch.
Roll two thirds of the dough into 6 circles 1/4 inch thick and line a 12-muffin jumbo tin. Place filling in each shell. Roll out remaining dough and top filling, pinching edges like a pie. Brush tops with egg wash and bake for 35 to 45 minutes. Let cool 30 minutes before removing from tin.