Preheat oven to 180°C (160°C for fan assisted ovens)/Gas Mark 4.
Put the butter, cream and sugar in a large saucepan and heat gently, stirring, until the butter has melted and the sugar dissolved. Simmer for 1 minute then pour into the base of a 33 cm x 22 cm rectangular cake tin. Scatter over the chopped pecans and top with the apple slices.
Mix the cake mix, oil, water and eggs together gently in a medium bowl and whisk (by hand or electric mixer) for 2-3 minutes, until smooth and creamy. Spoon the mixture over the apples and gently level the surface with a spatula.
Bake for 41-44 minutes until golden, risen and just firm to the touch. Leave to cool in the tin for 10 minutes then loosen the sides of the cake from the tin with a palette knife.
Invert the cake onto a heatproof serving platter. Leave the tin on the cake for 1 minute to allow the caramel to drizzle over the cake then remove the tin.
Serve warm, cut into pieces, each one topped with a spoonful of icing, chopped pecans and a drizzle of caramel sauce.