In a large frypan, heat oil over medium-high. Cook onion and garlic, stirring, until onion softens but does not brown. Add the silverbeet; cook, stirring, a further 3 minutes or until the silverbeet has wilted. Place mixture in a colander and allow to drain for a few minutes. Press down gently with a tea towel to remove all excess moisture.
Preheat oven to 200°C/180°C fan-forced.
In a large bowl, combine silverbeet mixture with cheeses, dill and nutmeg. Season, add eggs and mix well.
Place one sheet of pastry onto a greased oven tray and brush with melted butter. Repeat with 5 more sheets. Place the spinach mixture onto the pastry, leaving a 5cm border. Place another sheet of pastry on a clean board and brush with butter. Repeat with the remaining pastry, then place pastry sheets on top of spinach mixture. Roll up edges of pastry to seal. Brush top with butter. Bake about 40 minutes or until the pastry is golden.