Begin by finely chopping the onion and mincing the garlic, setting them aside for sautéing.
Heat a tablespoon of olive oil in a sauté pan over medium heat; once shimmering, add the chopped onion and cook, stirring occasionally, until translucent and fragrant, about 3-4 minutes.
Add the minced garlic to the pan and cook for another 30 seconds until fragrant, being careful not to burn it.
Transfer the sautéed onion and garlic into your slow cooker. Pour in the vegetable broth and stir to combine.
Next, add the greens—either fresh spinach or thawed frozen spinach—and stir until evenly distributed in the broth.
Set the slow cooker to low and cook for 4 to 6 hours, allowing the greens to wilt completely and flavors to meld, filling your kitchen with a fragrant aroma.
Once cooking is complete, carefully transfer the hot mixture to a blender or use an immersion blender directly in the slow cooker to puree until smooth and velvety, about 1-2 minutes.
Stir in the heavy cream or coconut milk for richness, then squeeze in fresh lemon juice to brighten the flavor. Season with salt and pepper to taste, blending again briefly to combine.
Taste the soup and adjust seasoning as needed, adding more lemon, salt, or pepper to suit your preference.
Serve the soup hot, garnished with a drizzle of olive oil or a sprinkle of fresh herbs if desired. Enjoy its creamy, vibrant green goodness that feels like a cozy hug in a bowl.
