Combine sugar and water in a saucepan and stir over medium heat until sugar dissolves, bring to the boil and simmer for five minutes.
Cool to room temperature, stir in lemon juice and rosewater.
Score skin of achacha and scoop out flesh. Remove flesh from seed. Combine with raspberries and toss. Divide mixture among six glasses, drizzle with syrup, top with yoghurt and sprinkle with pistachios.