Peel the sweet potatoes, then cut them into 3-cm chunks.
Chop the onion and mince the garlic.
In a large saucepan, heat the oil, then sweat the onion and sweet potatoes for 5 minutes. Add the garlic and oregano, stir for 1 minute.
Drain the beans, add to the sweet potatoes, then pour in the tomato sauce and the water (enough to cover the vegetables). Season with salt and pepper, then cook for 15 minutes.
When the sauce is thick, the sweet potatoes are tender, turn off the heat.
Crumble the feta on top and sprinkle with mint just before serving.