1. Reserve 6 small strawberries for decoration. Hull and roughly chop the rest and put them in a saucepan with the orange zest and juice, plus sugar. Stir over a gentle heat until the sugar has dissolved, then simmer for 5–10 minutes until the strawberries are soft and pulpy.
2. Mash with a fork and stir in the marshmallows until melted. Transfer to a bowl and set aside to cool.
3. Fold the fromage frais into the strawberry mixture and spoon into 6 serving glasses. Chill for at least 6 hours, until set.
4. Hull and slice the reserved strawberries and arrange on top of the mousses. Scatter over the pistachios to serve.