Sauté the Aromatics. Heat olive oil or butter in a large pot over medium heat. Add the chopped onion and garlic, sautéing until soft and fragrant. This step helps develop the rich base flavor of the soup.
Add the Broth and Spinach. Pour in the vegetable or chicken broth, then add the fresh spinach. Stir and let it cook for 3-5 minutes until the spinach is wilted and tender.
Blend Until Smooth. Use an immersion blender or transfer the mixture to a regular blender. Blend until you achieve a smooth, velvety consistency. If using a regular blender, be sure to allow the soup to cool slightly before blending to avoid splattering.
Add Cream and Seasonings. Return the blended soup to the pot and stir in the heavy cream (or milk). Add salt, black pepper, and a pinch of nutmeg for added depth. Simmer for a few more minutes to allow the flavors to meld.
Finish with Lemon Juice. Before serving, stir in a splash of lemon juice to brighten the flavors.
Serve and Enjoy. Ladle the warm spinach soup into bowls, garnish with extra cream, grated Parmesan, or fresh herbs, and serve immediately.