Peel the garlic cloves, cut in half lengthwise, and remove the central greenish core. Put in a small pan of salted water and bring to a boil. Drain and repeat twice to remove the garlic's heat and leave only the sweetness behind.
Heat the olive oil in a large, heavy pot over medium heat until hot. Add the onion, leek, celery, bay leaf, and a little salt to draw the water out of the onion. Cover and cook until the vegetables are soft but not brown, about 10 minutes. Add the potatoes. Add the blanched garlic and stock. Season with salt and pepper, to taste. Bring to a boil over high heat, reduce the heat to a simmer, cover and cook until the vegetables are very tender, about 15 minutes.
Remove soup from the heat and cool to room temperature. Place cooled pot with soup in refrigerator to chill. Day of party re-heat soup over medium heat. Add the oregano, spinach, and cream and cook 1 minute to wilt the spinach. Transfer to blender and carefully blend until smooth *see note below.
*When blending hot liquids: Remove liquid from the heat and allow to cool for at least 5 minutes. Transfer liquid to a blender or food processor and fill it no more than halfway. If using a blender, release one corner of the lid. This prevents the vacuum effect that creates heat explosions. Place a towel over the top of the machine, pulse a few times then process on high speed until smooth.