Lamb hotpot with merguez and butter beans

Ingredients

• 2 tbsp olive oil
• 500g lamb mince
• 8 merguez sausages
• 2 onions, finely chopped
• 2 celery sticks, finely chopped
• 1 carrot, finely chopped
• 4 garlic cloves, finely sliced
• 1 red chilli, finely chopped
• 1 tbsp ras el hanout
• 400g tin chopped tomatoes
• 600ml lamb stock
• 700g jar butter beans, drained and rinsed
• 800g maris piper potatoes, peeled and finely sliced (ideally 2.5mm thick using a mandoline)
• 60g unsalted butter, melted
• Greek yogurt and mint leaves to serve
• 200g cavolo nero, any tough bits discarded, sliced into strips
• 150g savoy cabbage, any tough bits discarded, sliced into strips
• Iced water
• 60g unsalted butter