Begin by washing and roughly chopping the fresh spinach, then set aside.
Dice the yellow onion finely, and mince the garlic cloves to release their fragrant aroma.
Add the olive oil to your slow cooker, then toss in the diced onion and minced garlic. Cook on low for about 30 minutes until they become fragrant and slightly translucent.
Pour in the vegetable broth, stirring to combine, and season with a pinch of salt and pepper. Cover and cook on low for 4 hours, letting the flavors meld and the onions soften.
About 30 minutes before the end, stir in the chopped spinach. It will wilt down and turn a vibrant, glossy green as it cooks.
Once cooking time is up, use an immersion blender directly in the slow cooker to blend the soup until smooth and velvety. Alternatively, transfer in batches to a blender, blending until silky, then return to the slow cooker.
Stir in the heavy cream or coconut milk to add richness, then taste and adjust seasoning with salt, pepper, and a squeeze of lemon juice for brightness.
Let the soup rest for a few minutes, then ladle into bowls. Drizzle with extra virgin olive oil or an additional squeeze of lemon if desired, and serve hot.
