Warm the olive oil and butter in a large saucepan or Dutch oven over medium heat. Add the green garlic and a pinch of salt, and cook, stirring frequently, until it is soft and translucent.
When the garlic is ready, add the stock, raise the heat a bit, and bring it to a boil. Then adjust the heat to maintain a gentle simmer, and continue to cook for about 15 minutes. Add the spinach, and immediately turn off the stove. Let it sit for 5 minutes – not too long, or the spinach will lose its color – and then, working in batches, purée the mixture in a blender. (Remember never to fill the blender more than a quarter or a third full, because the hot liquid will expand when you turn on the motor.) The soup should be a rich shade of green and very smooth.
Return the soup to the pot, and place it over low heat to re-warm gently. Add 1 Tbsp. crème fraîche or sour cream and another pinch or two of salt. Taste, and adjust seasoning as necessary.
Serve warm or hot, with a drizzle of olive oil or a dollop of crème fraîche or sour cream, if you like.