Dauphinoise galette - Good Food Middle East

Ingredients

• 1 tbsp butter
• ½ tbsp olive oil
• 1 large onion , finely sliced
• 3 medium Maris Piper potatoes , peeled (about 500g)
• 200ml double cream
• 50ml milk
• 1 garlic clove , crushed
• 5 thyme sprigs , leaves picked
• buttered greens , to serve
• 200g plain flour
• 100g cold butter , cubed
• 10g picked thyme leaves
• 1 tsp English mustard powder
• 1 egg
• 1 tbsp flour
• 200ml milk
• 1 bay leaf
• 150g mature cheddar
• freshly grated nutmeg
• 2 tsp Dijon mustard