Herby root vegetable gratin - Good Food Middle East

Ingredients

• 25g butter, plus extra for greasing
• 1 onion, thinly sliced
• 425ml double cream
• 300ml milk
• 1kg potatoes, thinly sliced
• 350g swedes, thinly sliced
• 350g parsnips, thinly sliced
• small bunch parsley
• 2 tbsp thyme leaves
• 1 garlic clove, crushed
• 50g Parmesan (or vegetarian alternative), grated
• 3 tbsp olive oil