In a large saucepan, gently soften the leeks and the carrots in the butter for about 3 minutes. Tip in the peppers and pour in the vegetable stock; season with salt and pepper and cook for 30 minutes on a medium heat.
Using a blender, whiz the soup until smooth. Return the soup to the pan, stir in the soured cream and reheat gently.
Slice a baguette to 1in thick. Toast on one side; then top with your favourite cheese on the other side. Grill until the cheese melts, before arranging over the soup and serving straight away.