Melt butter in a large soup pot over medium-low heat. Add onions and sauté until translucent, about 10 minutes. Add turnips, potatoes, and salt. Stir to combine, cover, and let cook for another 15 minutes. Add the stock and simmer 20 minutes or until veggies are soft. Puree with an immersion blender, or in batches in a blender or food processor. Serve hot!