Peel, quarter, core and chop apples into slices.
Put in a lidded pan with the lemon juice, water and sugar and gently cook for 5 – 10 minutes until completely softened and then mash with a fork.
Place the puree in a bowl to cool.
To make the Swiss Roll pre-heat the oven to 170⁰C.
Grease and line a Swiss Roll tin (24cm x 34cm) with greased baking paper.
Whisk the eggs and sugar together until mousse like and a trail of batter will sit on the surface for 15 seconds.
Sift in the flour and fold in with a large metal spoon.
Then fold in the boiling water.
Tip the batter into the tin and spread out evenly into the corners with a spatula.
Place in oven on the middle shelf for 10 minutes or until the centre springs back to the touch.
Once cooked tip out onto a sheet of baking paper (if you wanted to make this without the chocolate topping drizzle the paper with caster sugar beforehand).
Using a long sharp knife trim off the outer dry edges of the sponge.
To help with the initial rolling, score into the sponge 2/3 way down, 2 cm away from the short end nearest to you.
Spread the apple puree evenly over the sponge leaving a small gap either side to allow for spread (holding a little puree back if necessary).
Using the baking paper (we don’t want finger marks) as a guide, roll up the sponge onto itself from the scored end keeping it really tight until you reach the end which should be underneath.
Hold in this position for a minute then place along with the baking parchment onto a wire rack to cool.
Meanwhile make the chocolate topping by breaking up the dark chocolate and placing it in a bowl along with the butter over a pan of barely simmering water.
Once melted and combined pour the chocolate over the Swiss Roll and spread with a spatula to evenly cover.
Place the walnut halves along the top.
Now you can either serve straight away with the chocolate still molten or when it has set along with some whipped or clotted cream if you wish.
Keep in the fridge.