Beef stew with light herby dumplings

Name: Beef stew with light herby dumplings
Ingredients: spray oil, for frying; 600g/1lb 5oz lean braising steak, all visible fat removed; 1 litre/1¾ pints beef stock, made from 2 low-sodium cubes; 1 large onion, finely chopped; 2 garlic cloves, crushed; 2 celery sticks, finely chopped; 3 carrots, 1 finely chopped, rest cut into chunks; 1 tbsp tomato purée; 1 tbsp balsamic vinegar; 1 parsnip, cut into chunks; 300g/10½oz butternut squash (about half a squash); 150g/5½oz self-raising flour; pinch salt and freshly ground black pepper; 2 tbsp chopped fresh parsley or thyme (depending on preference); 1 egg yolk, lightly beaten; 1 tsp olive oil