Onion, Thyme & Sour Cream Tart - Sam Stern

Ingredients

• 110g plain white flour
• 115g wholemeal flour
• 110g chilled butter
• 25g cheddar
• 3 tbsp chilled water
• 1 egg white for sealing
• 600g onions
• 4 cloves of garlic, crushed
• 40g butter
• 1 tbsp oil
• 75g strong cheddar/gruyere
• 1 tsp fresh thyme
• 3 eggs
• 2 egg yolks
• 50ml sour cream
• 90ml double cream
• 1 tsp Dijon mustard
• seasoning