Melt the butter in a large pot over medium heat. Add the chopped onion and garlic, and sauté for 3-4 minutes.

Add the diced potatoes and cook for another few minutes, stirring occasionally.

Pour in the vegetable or fish stock, bring to a boil, and simmer for about 15 minutes until the potatoes are tender.

Add the milk or cream and blend the soup until smooth using an immersion blender.

Stir in the smoked salmon and cook for a few more minutes. Season with thyme, salt, and black pepper.

Serve hot in bowls, garnished with fresh dill or parsley.