Finely chop the onion, garlic, celery, carrot and red pepper. Peel and dice the sweet potato into small cubes.
Heat a splash of olive oil in a saucepan over low heat. Add the onion, carrot and celery, and gently fry for about 5 minutes until softened.
Stir in the red pepper, sweet potato and oregano. Cover with a lid and sauté for 10 minutes, stirring occasionally.
Add the tomato purée and stir to coat the vegetables well. Pour in the chopped tomatoes and enough water to cover. Add the stock cube.
Bring to the boil, then reduce to a gentle simmer. Cover and cook for 20 minutes until the vegetables are tender.
Stir in the fresh herbs, then turn off the heat. Use a stick blender to purée the mixture until smooth and creamy. Adjust the consistency with extra stock or water, if needed.
Meanwhile, cook the tortellini according to packet instructions (usually just 2–3 minutes). Drain gently with a slotted spoon.
Divide the sauce into bowls, top with tortellini and garnish with fresh herbs and/or grated Parmesan (vegan or traditional). Serve immediately.