Start by crushing the garlic cloves and finely chopping the onion; set aside to release their flavors.
Add the chopped onion and crushed garlic to the slow cooker, then pour in the vegetable broth. Cover and turn the slow cooker on low.
Let everything simmer gently for about 2 hours, until the onions are translucent and the garlic is fragrant, filling your kitchen with a warm aroma.
After about 1.5 hours, open the lid and stir in the fresh spinach in batches, gently folding it in so it wilts into the broth. Continue cooking for another 10 minutes until the greens are tender.
Use an immersion blender directly in the slow cooker to blend the soup until smooth and velvety, or carefully transfer the mixture to a blender in batches. Be cautious with hot liquids!
Stir in the heavy cream and a pinch of nutmeg, then season with salt and pepper to taste. Blend again briefly to combine all flavors and achieve a silky, vibrant green soup.
Ladle the soup into bowls, and let it rest for a few minutes so the flavors settle. Finish with a drizzle of cream or a sprinkle of herbs if desired, then serve hot.
