Sausage & winter greens cannelloni

Ingredients

• 1 tbsp olive oil
• 2 red onions, halved and sliced
• 6 pork sausages
• 3 garlic cloves, crushed
• small bunch of thyme, leaves picked
• pinch of chilli flakes
• 1 tbsp tomato purée
• 400g can plum tomatoes
• 200g cavolo nero or other winter greens
• 75g butter
• 75g plain flour
• 850ml milk
• nutmeg, for grating
• 50g parmesan, grated
• 12 lasagne sheets
• green salad, to serve