Preheat the oven to 190°C.
Line a 23cm tart tin with pastry and weigh it down with baking beans.
Bake the pastry case for 15 minutes.
Remove beans and paper then bake for another 10 minutes until golden.
Beat 175g butter and 175g sugar until light and fluffy.
Mix in 4 eggs one at a time.
Fold in the ground walnuts and breadcrumbs.
Spread 200g jam over the pastry, cover with walnut filling, and top with walnut halves.
Bake for 25-30 minutes until risen and golden-brown.
Whip double cream to soft peaks and fold in the remaining jam.
Preheat the oven to 200°C.
Fry medlars in 50g butter in an ovenproof pan for 2 minutes.
Add sugar and cinnamon then roast in the oven for 10-15 minutes until soft.
Serve tart slices with damson cream and roasted medlars.