Melt the oil in a saucepan over a medium heat and soften the onion for 5 mins. Add the garlic and ginger, and cook for 1 min until fragrant. Stir in the curry paste, peanut butter and sweet potato, then add the coconut milk and 200ml water. Bring to the boil, turn down the heat and simmer, uncovered, for 25-30 mins or until the sweet potato is soft.
Stir through the spinach and lime juice, and season well. Serve with cooked rice, if you like, and if you want some crunch, sprinkle over a few dry roasted peanuts.