1 onion, chopped
1 pound spinach, trimmed of thick stems
2 cups vegetable stock or water
2 cups cream, half-and-half, whole milk, or nondairy milk
Salt and pepper
1. Put the butter or oil in a large pot over medium heat. When the butter foams or the oil is hot, add the onion and cook, stirring, until it softens, 3 to 5 minutes.
2. Add the spinach and cook, stirring, until it’s coated and beginning to wilt. Add the stock, bring to a boil, then quickly lower the heat to a bubble. Cover and cook, stirring once or twice, until the spinach is tender but still brightly colored, just a minute or 2.
3. Use an immersion blender to purée the soup in the pot. Or let the soup cool a little, carefully purée it in a blender (working in batches if necessary), and return it to the pot. (You can make the soup in advance up to this point. Cool, cover, refrigerate for up to 2 days, and reheat before proceeding.) Stir in the cream and let heat up, stirring frequently, without bringing the soup to a boil. Taste, adjust the seasoning, and serve.