Tarte Bourguignonne

Pastry

• 85 g butter cold, diced, plus extra for greasing
• 250 g plain flour plus extra for dusting
• 1 tsp fine sea salt
• 100 g water ice cold

Filling

• 100 g Gruyère cheese cut in pieces
  of 100 g Cheddar cheese cut in pieces
• 300 g onions quartered
• 30 g olive oil
• 1 tsp fine sea salt
• 400 g beef mince
• ¼ tsp ground black pepper

White Sauce

• 40 g plain flour
• 40 g unsalted butter diced
• 400 g milk
• 1 pinch fine sea salt
• 1 pinch ground black pepper