In a 2- to 3-quart pan over medium-high heat, stir onion and garlic in extra-virgin olive oil until onion is barely limp, 2 to 3 minutes. Add spinach leaves and stir just until wilted, about 1 minute. Add vegetable stock, or use canned broth. Purée mixture, one half at a time, in a blender (hold top down with a towel). Return to pan and bring to a simmer. Add sea or kosher salt and pepper.