Heat oven to 180C/160C fan/gas 4. Toss the potatoes with half the oil and some seasoning on a baking tray. Bake for 30 mins, tossing halfway through cooking, until tender. Meanwhile, heat the remaining oil in a pan, add the onion and cook for 5 mins until soft, then add the garlic, chilli and coriander stalks. Cook everything for a further 2-3 mins, stirring to prevent the garlic from burning. Sprinkle in the spices, stirring for 1 min more, until aromatic. Stir in the chipotle paste, yeast extract, tomatoes and half a can of water, swirling it around the tin to wash out all the bits of tomato. Simmer the sauce, uncovered, while the sweet potato is cooking, adding a splash more water if it looks too dry.