Gently melt 30g/1oz butter in a large frying pan and sauté the apples for 3-4 minutes.
Add the sugar, cinnamon and nutmeg to the pan and continue to cook for a further 2-3 minutes.
Stir in the walnuts and remove from the heat.
Using a large cutter, cut a round out of one slice of bread.
Gently melt the remaining butter in an ovenproof frying pan and add the bread round (with the ring) to the pan.
Line the ring with the remaining slices of bread.
Beat the mascarpone and egg yolk together in a medium bowl.
Stir the sautéed apples into the mascarpone mixture until combined.
Spoon the mixture into the ring with the bread then transfer to the oven and bake for 8-10 minutes, or until the bread is toasted.
Remove from the oven, transfer to a serving plate and remove the ring.
Place the cream in a bowl and whisk until thickened. Spoon the whipped cream onto the top of the baked apple charlotte and sprinkle with the orange zest.