1. Heat oil in large frying pan, add onion and garlic; cook, stirring, until soft. Add lamb and oregano; stir until cooked through, season to taste.
2. SILVER BEET AND CHEESE FILLING: Finely slice silver beet stems and leaves separately. Heat oil in large frying pan, add stems; cook, stirring, until tender. Add leaves; cook, stirring, until soft. Remove from heat; stir in ricotta, fetta, rind and mint, season to taste.
3. Preheat oven to 220°C (200°C fan-forced). Brush 20cm loose-based square cake pan with a little of the butter. Layer three pastry sheets, brushing each sheet with butter. Place in cake pan to cover base and two opposite sides with edges overhanging. Brush top pastry sheet with butter. Repeat with three more sheets, placing crossways over sheets in pan.
4. Spread half the lamb mixture over pastry base, then half the filling; repeat with remaining lamb mixture and filling. Fold overhanging pastry over filling. Layer remaining sheets of pastry, brushing each with butter, fold to fit top of pie. Brush with butter.
5. Bake about 25 minutes or until pastry is browned lightly. Serve hot or at room temperature; accompany with lemon wedges. Serve with a green salad, if desired.

Unbaked spanakopita suitable to freeze. Not suitable to microwave.