Soula's Spanakopita

Dough

• 20 g olive oil, plus extra to grease
• 525 g lukewarm water
• 1 tsp sea salt
• 900 g baker's flour, plus extra to dust
• 100 g unsalted butter, cut into pieces

Filling

• 2 - 3 bunches fresh English spinach, trimmed, leaves and stalks cut into pieces (approx. 5 cm - 550 g - see Tips)
• sea salt, to taste (optional)
• 1 onion, cut into halves
• 500 g feta cheese, cut into pieces (4 cm - see Tips)
• 1 egg
• olive oil, to drizzle