Cook the lentils according to the instructions on the package. Then drain, but leave a little liquid.
Cut the sweet potato into approx. 1 cm cubes.
Sauté the onion, ginger and garlic cubes in 2 tablespoons butter, then add the sweet potato cubes. Fry briefly while stirring and add the stock. Let everything simmer for about 10 minutes. Add the lentils and let steep for another 5 minutes on low heat.
Meanwhile, caramelize the barberries in 2 tablespoons of butter and the sugar. Add to the stew and season everything with the spices (except for the fresh coriander).
Arrange on plates and serve decorated with a good tablespoon of sour cream and some fresh coriander.