Sift the flour and baking powder into a large mixing bowl. Add the ground almonds and sugar and stir until evenly combined. Meanwhile, melt the butter in the microwave on high for 1 minute.
Add the melted butter, vanilla and eggs to the bowl and beat, using a handheld electric whisk, until smooth. The mixture will be slightly thicker than a standard cake mix.
Spoon the mixture into a 1-litre microwave-proof glass dish. Cook for 2 minutes at 600W. Arrange the nectarine pieces on top of the cake mix, cut-side up, and lightly press into the sponge. Cook for 3 minutes at 600W until cooked through – you may need to return it to the microwave for a further minute. Remove from the microwave and allow to cool slightly.
Meanwhile, heat the jam in the microwave on high for 30 seconds, then brush over the nectarines and sponge. Scatter over the flaked almonds, dust with icing sugar and serve immediately.