Potato and mushroom pie

Ingredients

• 50 g butter, chopped
• 1 large onion, chopped finely
• 2 garlic cloves, crushed
• 1 kg mixed mushrooms, sliced
• 1/2 cup dry white wine
• 1 cup finely grated gruyere cheese
• 1 cup finely grated parmesan
• 1 tbsp finely chopped rosemary, plus 1 tablespoon rosemary sprigs to serve
• 1 kg small desiree potatoes, unpeeled, cut into 5mm-thick slices
• 1/3 cup milk
• 300 ml thickened cream