Cheesy potato patchwork pie - Good Food Middle East

Ingredients

• 500g Maris Piper potatoes , unpeeled, cut into 1 cm thick slices
• 1 tbsp neutral oil like rapeseed or sunflower
• 1 tbsp butter , plus extra for the dish
• 500g leeks , halved and sliced
• 2 garlic cloves , finely sliced
• 5 thyme sprigs
• 2 bay leaves
• 1 tbsp plain flour
• ½ tsp ground mace , or a pinch of ground nutmeg
• 1 tbsp Dijon mustard
• 300ml milk
• 50g parmesan or vegetarian alternative, grated
• 100g extra mature cheddar , grated
• steamed greens or watercress salad, to serve (optional)
• 250g plain flour , plus extra for dusting
• 125g salted butter , cold and cut into cubes
• 1 whole egg and 1 egg yolk (freeze the egg white for another recipe)
• 1 tbsp milk
• 5 thyme sprigs , leaves picked