Put the oil in a medium Dutch oven over medium-high heat. Fry the onions until translucent, lower the heat to medium if necessary. Add the spices and stir. Add sweet potatoes and potatoes and saute them for 1-2 minutes. Add the rest of the ingredients, cover the vegetables with water and bring to a boil. Boil for 30 minutes then lower the heat to low and cook for 1 hour.
Preheat the oven to 350°F / 180°C. Taste the broth and corect seasoning.
Cover the pot and bake for 3 hours. Check the liquid level – the vegetable and broth should be the same height – then lower the temperature to 250°F or 120°C and bake overnight or minimum 6 hours.
Before you go to bed check again the liquid level, if necessary add a little bit of water to prevent burning the bottom.