Slow cooker chicken casserole with dumplings

Name: Slow cooker chicken casserole with dumplings
Ingredients: 8 chicken thighs, skin on, bone in; 2 tbsp sunflower oil; 2 onions, thinly sliced; 600g/1lb 5oz large waxy potatoes, peeled, cut into chunks; 3 carrots, peeled, cut into chunks; 2 parsnips, peeled, cut into chunks; 3 tbsp plain flour; 200ml/7fl oz hot chicken stock (made with 1 stock cube); 500ml/18fl oz dry cider; 1 tbsp wholegrain mustard; salt and freshly ground black pepper; 200g/7oz self-raising flour, plus extra for dusting; 100g/3½oz suet, shredded; 3 tbsp finely chopped fresh sage (or 1 tsp dried sage leaves)