Place the biscuits, egg yolk, butter and sugar into a food processor and pulse until well combined.
Place the nectarine halves on a baking tray, cut side up, and spoon the biscuit mixture inside. Bake in the oven for 6-8 minutes, or until the nectarines are tender and the stuffing is golden-brown and crisp.
Whip the mascarpone cheese together with the cream until smooth and creamy. Using two spoons, form the mixture into quenelles and place on top of the nectarine halves. Garnish with a sprig of mint.