Spray a large frying pan with low-calorie cooking spray and place over a medium heat. Add the onion, leek and garlic and cook for 6-8 minutes until softening and golden, then transfer to the slow cooker.
Add the butternut squash, carrots, mushrooms and cannellini beans to the slow cooker. Add the stock, mustard powder, bay leaves and thyme, and stir to combine. Cover with the lid and cook on high for 2½- 3 hours, until the butternut squash and carrots are tender. Some butternut squash chunks will break up to thicken the stew.
While the stew is cooking, make the dumplings. Sift the flour and baking powder into a small bowl. Add the egg yolks and mix slightly with a fork. Using your fingertips, rub in the egg yolks until the mixture resembles breadcrumbs.
Add the Parmesan, parsley, mustard powder, garlic granules and season with salt and black pepper. Stir well. 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 will need will depend on the size of the egg yolks. Shape into four equally sized dumplings and set aside in the fridge for later.
When the stew has been cooking for 2½- 3 hours, and the vegetables are tender, remove the bay leaves and discard. Stir in the spinach leaves for a minute or two until just wilted.
Put the dumplings on top of the stew and replace the lid. Continue to cook on high for a further 40 minutes, or until the dumplings have puffed up and are cooked through.
Remove the dumplings from the stew and set aside on a plate. Add the cream cheese to the stew and stir in until evenly blended. Taste and season well with salt and black pepper if needed.
Put the cooked dumplings back on top of the stew, sprinkle with a little chopped curly parsley (optional) and serve at once. If you would like the dumplings to be golden, transfer the stew and dumplings to an ovenproof dish and place under a preheated grill for a few minutes. Remove from under the grill and sprinkle with chopped curly parsley if using.