Kale and green lentil soup

Name: Kale and green lentil soup
Cuisine: Middle Eastern
Ingredients: 2 tsp sunflower oil; 2 onions, finely chopped; 1 large carrot, diced; 1 stick celery, chopped; 1-2 cloves garlic, crushed; 1 tsp ground cumin; 1 tbsp tomato purée; 1 x 400g can green lentils in water (add the water, too); 200g Cavolo Nero (black kale) or curly kale, chopped (thick stalks removed); 500ml vegetable stock; black pepper, to taste