Vegetarian Borscht
1 onion, chopped 2 carrot, coarsely grated 2 small beets, peeled and coarsely grated 2 tablespoons chopped fresh dill 2 tablespoons sour cream, or more to taste 2 tablespoons tomato paste 2 tablespoons vegetable oil, or as needed 3 medium potatoes, peeled and diced 3 tablespoons vinegar 8 cups water salt and pepper to taste ¼ medium head cabbage, shredded ¾ cup dry yellow lentils