Slow-cooker sausage casserole

Ingredients

• red onions , finely chopped
• celery stick , finely chopped
• rapeseed oil
• carrots , cut into fat pieces
• chipolatas , each halved
• sweet potato , peeled and cut into chunks
• tin tomatoes
• tomato purée or tomato and veg purée
• thyme sprig
• rosemary sprig
• beef stock cube or stock pot