Peel the sweet potatoes and chop them into uniform 1-inch cubes. Dice the onion and mince the garlic.
Heat the olive oil in a skillet over medium heat until it shimmers. Add the diced onion and minced garlic, sautéing until fragrant and translucent, about 5 minutes. You'll notice a golden hue and a warm aroma filling the kitchen.
Transfer the sautéed aromatics to the slow cooker. Add the chopped sweet potatoes and pour in the vegetable broth. Stir in the cumin, cinnamon, and chili powder for depth of flavor.
Set the slow cooker to low and cook uncovered for 6 hours, until the sweet potatoes are very tender and easily pierced with a fork. The house will fill with a fragrant, spicy aroma as the flavors meld.
Once cooked, use an immersion blender directly in the slow cooker to blend the soup until smooth and velvety. If using a regular blender, carefully blend in batches, then return to the pot. The soup should be bright orange and silky, with a fragrant, spiced aroma.
Stir in the coconut milk for added richness and flavor. Taste and season with salt and pepper, adjusting to balance the sweetness and spices. The soup should shimmer with a slight sheen and smell warm and tropical.
Ladle the hot soup into bowls, garnish with herbs if desired, and serve immediately. The texture should be smooth and silky, with a vibrant orange hue and a cozy, inviting aroma.
