Preheat the oven to 300°. Lay the walnuts for the filling and tart pastry (2 cups total) onto a baking sheet and toast for 10-12 minutes or until lightly golden and fragrant. Set aside 1/2 cup of toasted walnuts for the crust.
Roughly chop the remaining 1 1/2 cups of nuts for the filling and set aside.
MAKE THE FILLING:
Line a 9” pie pan with plastic wrap and set aside.
In a small saucepan heat the sugar and cream to boiling, stirring occasionally until the sugar is dissolved and the cream thickens slightly.
Add the walnuts, cranberries and chopped apple to the cream mixture and stir to combine. Transfer the apple mixture to the prepared pie pan and spread the apple walnuts into an even layer on the bottom of the pan.
Loosely cover and refrigerate for at least 2 hours or overnight. The nuts will absorb some of the liquid and the mixture will set.
MAKE THE WALNUT PASTRY
Add the 1/2 cup of toasted walnuts to a food processor and pulse until finely chopped. Add the flour, sugar and salt and pulse to combine.
Add the butter to the flour mixture and pulse until it resembles coarse meal with some larger pieces of butter.
In a small bowl, mix the egg yolk with 3 tablespoons ice water and whisk together. Add the egg yolk mixture and pulse about 8 times or until the dough starts to come together.
Transfer the pastry dough to a workspace lined with a piece of plastic wrap. Bring the dough together, kneading 3-4 times. Form it into a ball and flattening it into a disc. Wrap the pastry with plastic wrap and chill for 20 minutes. If you chill it overnight, remove the pastry 20 minutes before rolling it out so that it doesn’t tear.
ASSEMBLE THE TART:
Place the rack in the center of the oven. Preheat the oven to 375°.
Line a workspace with plastic wrap. Place the pastry disc in the center of the plastic wrap and cover with another piece of plastic. Roll the pastry out to a 12-14" circle.
Remove the top sheet of plastic wrap and discard. Transfer the pastry to the tart pan by peeling up the plastic wrap beneath the pastry and carefully lifting the pastry off the counter. Flip the pastry into the tart pan and discard the plastic wrap.
Gently press the pastry into the bottom and sides of the tart pan, then roll the rolling pin over the edges to trim additional crust from the tart and give a clean edge.
Remove the tart filling from the refrigerator and invert it into the tart shell. Use a rubber spatula to smooth any areas that are uneven.
Carefully transfer the tart pan with two hands to a large rimmed sheet pan to catch any drips (and to make it easier to carry).
In a small bowl combine the cinnamon and sugar for the topping and sprinkle liberally over the tart. Dot the tart with butter and bake for 30-35 minutes or until the crust is golden brown.
Let the tart cool for 15 minutes before slicing. Best served warm with vanilla or cinnamon ice cream.
This tart will keep at room temperature for 2 days and can be reheated in a 350° oven for 5 minutes.