Beetroot, feta and spinach quiche

Pastry

• 200 g buckwheat groats
• 1 pinch salt
• 100 g unsalted butter, chilled and cut into pieces
• 45 g water
• plain flour, for dusting

Filling

• 100 g brown onion, cut into halves
• 1 garlic clove
• 250 g raw beetroot, peeled and cut into quarters
• 5 eggs
• 400 g full cream milk
• 1 - 2 pinches salt
• 1 - 2 pinches ground black pepper
• 50 g fresh spinach leaves, torn into pieces
• 100 g feta cheese, cut into pieces (1 cm)