Roughly chop the onions. No need to be precise as everything will be blended.
Wash the vegetables. Peel the sweet potato and the carrots and cut them into relatively small pieces (they will cook more quickly). If using fresh tomatoes, cut them into quarters.
Heat a large pot with olive oil. When hot, sauté the onion until translucent.
Add in the carrots and the sweet potato. Sauté for 5 minutes until lightly colored.
Add in the tomatoes (with their juice if using canned) and the vegetable broth. Simmer over medium heat with a lid on for 20 minutes or until the carrots and the sweet potatoes are soft.
Blend the soup with a stick blender. Add more vegetable broth or water if you prefer a thinner consistency.
Season with salt and pepper. Serve with some fresh thyme and chili flakes if you like it spicy.