Chop the onion finely and mince the garlic cloves. Sauté them in a skillet over medium heat with butter or olive oil until fragrant and golden, about 5-7 minutes. The onions should be translucent and the garlic aromatic.
Transfer the sautéed onion and garlic into your slow cooker. Pour in the vegetable broth and add the chopped spinach. Season lightly with salt and pepper.
Set the slow cooker to low and cook for 4 to 6 hours, until the spinach is wilted and the broth is deeply flavored. The house will fill with a fresh, leafy aroma as it simmers.
Once cooked, use an immersion blender directly in the slow cooker to blend the soup until smooth and velvety. Alternatively, transfer the hot mixture in batches to a blender, blending carefully with the lid slightly ajar.
Stir in the heavy cream or coconut milk for richness. Taste the soup and add more salt, pepper, or a squeeze of lemon juice if desired to brighten the flavor.
Let the soup rest for 5 minutes to thicken slightly and allow flavors to meld. Give it a gentle stir, then serve hot with optional toppings like a drizzle of cream or a squeeze of lemon for extra flavor.