Peel, wash and cut the sweet potatoes into large pieces. Clean the oyster mushrooms and cut them in half or quarters depending on their size. Remove the outer leaves of the lemon grass, cut the stem in half lengthwise. Wash and dry lime leaves. Peel onion and garlic. Cut onion into strips, chop garlic finely.
Heat the oil in a large pot. Sear the mushrooms in it, take them out. Sauté onion in hot frying fat. Fry the garlic briefly. Add curry paste, lemongrass and lime leaves and sauté briefly. Add coconut milk, 1⁄4 l water and sweet potatoes, bring to the boil. Cook over medium heat for about 15 minutes. Add frozen beans after about 5 minutes and cook. Finally, briefly heat up the mushrooms again.
Clean and wash the chillies and cut them into rings with seeds. Thai basil wash and shake dry, pluck the leaves. Season curry with soy sauce, lime juice and sugar. Remove lemon grass and lime leaves if necessary. Stir in chilli and Thai basil. Sprinkle sweet potato curry with fried onions. Basmati rice tastes good with it.