In a large pot, melt the butter over medium heat. Add the chopped onion and sauté until translucent, about 5 minutes. Stir in the minced garlic and cook for an additional minute until fragrant.

Pour in the vegetable broth and add the diced potato. Bring to a boil, then reduce the heat and simmer for about 10 minutes, or until the potatoes are tender.

Add the chopped sorrel leaves to the pot and cook for another 5 minutes, until the sorrel is wilted and tender.

Remove the pot from heat and use an immersion blender to puree the soup until smooth. If you don’t have an immersion blender, carefully transfer the soup in batches to a regular blender.

Return the pureed soup to the pot and stir in the heavy cream. Heat gently over low heat, but do not boil. Season with salt, pepper, and lemon juice to taste.

Serve hot, garnished with fresh dill or parsley.