1. Melt the butter in a medium saucepan over medium heat. Add onion and saute until translucent, about 5 minutes. Add flour and cook, stirring constantly, for 1 minute (the flour should not take on any color).

2. Pour in stock, whisking to incorporate fully, and bring to a boil. Reduce to a simmer and cook for 10 minutes, whisking fairly often, to cook out raw flour taste and thicken the liquid (it should reach the consistency of heavy cream).

3. Blanch spinach: Wash well and trim tough ends while bringing a large pot of water to a boil. Prepare an ice-water bath. Salt the boiling water generously and submerge all of the spinach. Blanch 30 seconds, just until wilted and bright green, then remove spinach with a spider or slotted spoon and immediately plunge into the ice-water bath to stop the cooking, stirring to cool more quickly. Squeeze spinach with your hands to remove excess liquid.

4. Working in batches (do not fill more than halfway), add spinach and veloute base into a blender and puree until smooth. (Alternatively, puree soup in a food mill, using either fine or coarse disk, a food processor, or with an immersion blender.) Pass soup through a fine sieve into a clean pot, pressing on solids with a flexible spatula to extract as much liquid as possible.

5. Set pot over low heat and whisk in cream, if using. (Thin with more stock or water, if necessary.) Cream soups are the best made just before serving, especially when using green vegetables, as the soup can lose some of its vibrant color. You can make the veloute base the day before; let cool completely then refrigerate in an airtight container.

6. Heat oil in a small skillet over medium heat. Add bread cubes and cook until crisp and golden brown.

7. Serve soup garnished with croutons.