Preheat your oven to 400°F (200°C). Toss the diced sweet potatoes and chopped carrots with olive oil, cinnamon, and thyme, spreading them evenly on a baking sheet. Roast for about 25-30 minutes until tender and caramelized around the edges, filling your kitchen with a sweet, fragrant aroma.
Transfer the roasted vegetables to the slow cooker, adding the vegetable broth. Stir to combine and set the cooker to low. Let everything cook together for 4-6 hours, allowing the flavors to meld beautifully and the vegetables to become comfortably soft.
Once the cooking time is up, take an immersion blender or carefully transfer the mixture to a regular blender in batches. Blend until the soup is smooth and velvety, with a vibrant orange hue. Be cautious with the hot liquid, blending in steady pulses.
Return the blended soup to the slow cooker or a pot on the stove. Stir in the citrus zest for a bright, fresh note. Season with salt and pepper to taste, adjusting the flavors as needed and warming it gently until heated through, about 5 minutes.
Serve the soup hot, garnished with a squeeze of lemon or orange juice if desired. The final dish should be smooth, thick, and radiant in color, with a subtle hint of spice and citrus brightness on the nose.
