French Onion Soup Pie (for Lovers) Recipe | Winter Onion Pie

Ingredients

• 1 tbsp butter
• A glug of olive oil
• 500g onions, peeled and finely sliced
• ½ tsp salt
• ½ tsp ground black pepper
• ¼ tsp sugar
• 1½ tbsp plain flour
• 450ml boiling beef stock (made from 1 stock cube)
• 1 tsp wine vinegar
• 1 tbsp Cognac
• 150g puff pastry
• 30g Swiss or Gruyère cheese slice
• 1 egg, whisked with 1 tsp cold water, for egg-washing and glazing