Preheat oven to 220°C/200°C fan-forced. Line 3 baking trays with baking paper.
Cook spinach in a saucepan of boiling water for 1 minute or until just wilted. Drain. When cool enough to handle, squeeze out excess liquid. Finely chop spinach.
Meanwhile, heat oil in a large frying pan over medium-high heat. Cook onion, stirring, for 5 minutes or until softened. Remove from heat.
Combine ricotta and feta in a large bowl. Add onion, spinach, mint and 1 egg, then stir until well combined. Season with pepper.
Cut each pastry sheet into 4 squares. Place 2 tablespoons of the spinach mixture onto the centre of each square. Brush edges with water. Bring each corner of pastry squares into the centre to form a parcel, pressing edges to seal. Place on trays.
Beat remaining egg in a small bowl. Brush pastry with egg. Bake for 30 minutes or until golden and cooked through. Serve.