Serve this silky-smooth soup as a starter, or pair it with crusty bread for a light meal.
Melt butter in a large saucepan over medium heat. Add onion, garlic, and potatoes; stir to coat. Cook, stirring, 2 minutes.
Pour sherry, chicken broth, and 2 cups water into pan; stir to combine. Bring to a boil. Reduce heat to medium-low; cover, and simmer until potatoes are very tender, about 15 minutes.
Stir spinach into pan, and cook until wilted and bright green, about 3 minutes. Remove from heat. Puree soup with an immersion blender until smooth. (Alternatively, use a regular blender, working in batches so as not to fill jar more than halfway. Return mixture to saucepan.) Season with salt and pepper, and serve.