In a large soup/stock pot, melt the butter over medium heat. Add the onions and saute until translucent, 3-4 minutes. Add the garlic and saute for another 1 to 2 minutes – you want the garlic to soften but not brown. To that mixture, add the peeled potatoes. Stir to coat and begin softening them for 2 to 3 minutes. Add the vegetable broth (or water) and reduce heat to a simmer. Cover and cook the potatoes until fork tender – around 12 to 15 minutes. Add in the spinach and cook until just wilted. In a Vitamix/blender (you’ll have to do this in 2 to 3 batches but the texture is amazing when using a Vitamix or high powered blender) or using an immersion blender, puree the soup to desired consistency. Return to the soup pot and stir in the sherry, cayenne pepper, nutmeg and salt & pepper to taste. Before serving, bring the soup back up to temperature over medium heat. Garnish with parmesan cheese and other desired greens and serve immediately.