Silky Spinach Soup

Name: Silky Spinach Soup
Ingredients: 2 tablespoons unsalted butter; 1 medium yellow onion, diced; 1 large clove garlic, minced; 1 medium russet potato, peeled and cut into even 1/2″ chunks; 4 cups good quality vegetable broth or 2 cups vegetable broth & 2 cups water; 1/6 or 8 ounce bag of prewashed & dried baby spinach leaves; 1 small pinch (1/8 tsp) cayenne pepper or dried red chili flakes; 1 pinch fresh ground nutmeg; 2 tbsp. dry sherry; salt & fresh ground black pepper to taste; grated Parmesan cheese, flat leaf parsley, watercress – for optional garnish