Melt the butter in a large pan.

Add the diced potato, carrot, chopped onion, sliced leek, and sliced celery (if using) to the pan.

Cook the vegetables for about 5 minutes, stirring occasionally, until slightly softened.

Stir in the curry powder and cook for an additional 2 minutes, allowing the spices to become fragrant.

Add the tomato paste and vegetable broth to the pan.

Bring the soup to a simmer, then cover and cook for 20 minutes, or until the vegetables are tender.

Carefully transfer the soup to a liquidizer or use a hand blender to blend until smooth.

Season the soup with salt to taste.

Serve hot.