Place the puff pastry slices next to each other and defrost. Preheat the oven (electric cooker: 200 °C/circulating air: 175 °C/gas: see manufacturer). Line a baking tray with baking paper.
Stir mascarpone, lemon juice and vanilla sugar until smooth. Roll out puff pastry slices larger (approx. 20 x 30 cm). Halve the slices and place them on the baking tray. Spread thinly with mascarpone, leaving a rim of about 1 cm. Mix milk and egg yolk. Spread the edges with egg milk and sprinkle with brown sugar. Bake in a hot oven for 16-20 minutes.
Wash the nectarines, dab dry, halve and stone. Cut the flesh into slices. Caramelise 50 g sugar in a pan and carefully add 4 tbsp. water. Bring the caramel to the boil and simmer the nectarines in it at low heat for 2-3 minutes. Take out. Remove the tartes from the oven and spread nectarines on top. Let the tartes cool down a little. Serve with vanilla ice cream and the rest of the caramel.