In a large saucepan, heat the butter over medium heat. Add the diced onions and cook for 8 minutes until fragrant and lightly "caramelized". Sprinkle with salt. Meanwhile, prepare the mushrooms and garlic.
Add the sliced mushrooms and garlic and cook for 5-10 minutes or until the water from the mushrooms has evaporated.
Add the bay leaves and thyme.
Add the Worcestershire sauce and vegetable broth and bring the soup to a boil, then reduce to medium heat and simmer for 15 minutes.
Meanwhile, prepare the dumplings: In a medium bowl, mix the flour with salt and baking powder. Add the milk and mix the ingredients together to make a sticky dough. Set aside.
Transfer the soup to a high-speed blender or use an immersion blender, making sure to discard the bay leaves and thyme. Blend until smooth (or you can keep some texture). Transfer back to the saucepan, then stir in the milk. Top with little spoonfuls of dumpling dough. Cover and cook over medium heat for 5 minutes or until the dumplings are cooked through.