Peel the ginger and onion and dice them very small. Clean and wash the Brussels sprouts and cut in halves or quarters depending on the size. Heat coconut milk in a saucepan. Let the ginger and onion cubes simmer for 1 minute. Stir in the curry powder and pour in the vegetable stock. Add the Brussels sprouts and cook in a covered saucepan over medium heat for 10-15 minutes until al dente. Season Brussels sprouts soup with salt, pepper and lemon juice.