Heat oven to 180C/160C fan/gas 4. Sit the nectarine halves snugly in a baking dish, cut-side up. Put the amaretti biscuits into a large bowl and use the end of a rolling pin to bash into crumbs. Add the softened butter, ground almonds, caster sugar and egg, and stir together.
Push spoonfuls of the mixture into the cavities of the nectarines, piling more on top until the mixture is evenly divided among them. Scatter with almonds, then carefully pour the Marsala into the dish through a gap between the fruit so the topping doesn’t get soggy. Bake for 40-50 mins until golden and crisp, and the fruit is soft. Eat warm with the juices spooned over, and a dollop of Greek yogurt or creme fraiche alongside.