Place the sugar and halved plums in a saucepan and gently heat until the sugar has dissolved and the plums just start to collapse. In a large pot, bring the milk to a boil. Meanwhile, in a large mixing bowl, whisk the egg yolk and cornstarch together. Add the cooked plums and sugar to the boiling hot milk, then add the milk and plum mixture to the whisked egg yolk in one go, while whisking vigorously. Add the cream and whisk through. Leave to cool completely. Churn in an ice-cream maker according to manufacturer instructions.
For the Meringue
Grind the sugar and sage leaves together using a spice grinder or mortar and pestle. Whisk the egg whites and infused sugar together until thick and glossy. Spread thinly onto silicone baking mats and cook at 100°C for approximately one hour, until crisp when cooled to room temperature. Store in an airtight container.
Serve scoops of plum ice cream with slices of fresh plum, shards of wild sage meringue and wild sage flowers.