Peel and chop the sweet potatoes and carrots into roughly 2 cm pieces, setting them aside.
Heat the olive oil in a large pot over medium heat, then add the chopped onion if using. Cook for about 5 minutes until it becomes translucent and fragrant, filling your kitchen with a sweet aroma.
Add the chopped sweet potatoes and carrots to the pot. Stir well and cook for another 3-4 minutes, allowing the edges to soften slightly and release their earthy scent.
Pour in the vegetable broth, making sure it covers the vegetables. Bring the mixture to a gentle simmer and cook uncovered for 20 minutes, or until the sweet potatoes and carrots are fork-tender and easily mashable.
Use an immersion blender directly in the pot, or carefully transfer the mixture to a blender in batches. Puree until the soup is smooth, silky, and vibrant orange, about 1-2 minutes.
Return the blended soup to the pot if using a regular blender, then season with salt and pepper. Squeeze in a little lemon juice to brighten the flavors, and let it simmer gently for another 2 minutes.
Taste the soup and adjust the seasoning as needed. Once ready, turn off the heat and let the soup rest for a few minutes to deepen its flavor.
Ladle the hot, velvety soup into bowls. Optional garnishes like a drizzle of olive oil or fresh herbs can add extra flavor and visual appeal.
Enjoy your warm, nourishing bowl of sweet potato and carrot soup, perfect for cozy days.