Spray a large frying pan with low-calorie cooking spray and place over a medium heat. Add the onion, garlic and carrots and cook gently for 10 minutes until softening and golden.
Add the minced beef and break it up with a wooden spoon. Cook for 4-5 minutes until browned on all sides.
Transfer the beef and vegetables to the slow cooker. Add the stock, tomatoes, Worcestershire sauce or Henderson's relish, tomato puree and mixed herbs. Cover and cook on high for 2½-3 hours until the carrots are tender, then season with salt and freshly ground black pepper, to taste.
Sift the flour and baking powder into a small bowl. Season with a little salt and black pepper, then add the egg yolks and mix slightly with a fork. Using your fingertips, rub in the egg yolk until the mixture resembles breadcrumbs.
Add 3-4 tbsp of cold water, a little at a time, and stir with a round-bladed knife until it comes together to form a dough. You will need a firm dough that's not too wet, so the amount of water you need will depend on the size of your egg yolks. Shape the dough into four equally-sized dumplings and set aside in the fridge for later.
After the mince has been cooking for 2½-3 hours, place the dumplings on top and replace the lid. Cook for a further 40 minutes until the dumplings have puffed up and the mince and gravy is thick and rich. Serve.