Preheat large slow cooker (approx 6L) to high. Season chicken with salt and pepper. In a frypan over a high heat, add the oil and cook the chicken until golden brown and then transfer into the slow cooker.
Turn frypan down to medium heat. Add the diced bacon and cook for a minute or two to render out some fat. Then add the leeks and 1 tbsp butter. Season the leeks with a pinch of salt and add the thyme leaves. Cook until the leeks are tender and then add the garlic. Cook for a further minute or two or until garlic is fragrant and transfer to the slow cooker
Add the stock to the slow cooker. Give everything a mix and cook on high for 3.5 hours or on low for 5. Then add the cream to the slow cooker.
Mix together the water and corn flour to create a slurry and add it to the slow cooker. Mix it well. At this point I usually give the casserole a taste for seasoning.
In a bowl, add the self raising flour, baking powder, salt and butter. Rub the butter into the flour until it resembles sand almost. Add the chopped rosemary and mix through before adding the milk. The dumpling dough should be quite sticky/wet, if not then add 1 tbsp extra milk at a time until you achieve this. Divide the dough into 12 portions and set aside.
Set the slow cooker onto high if you haven't already done so and add the portions of dumpling dough onto the surface of the casserole, ensuring they have enough space between them to expand. Cook for 1 hour or until a toothpick comes out clean when inserted into a dumpling. Sprinkle fresh parsley on top and serve!