Dead easy.
Water, about 750ml
200gm of red lentils.
70gm of tomato purée
tin of mixed beans
1/4 each of red, green and yellow peppers sliced and diced
salt and pepper to taste
Cover the lentils in about 600ml of the cold water and bring to the boil. How much you soften the lentils is a bit of a personal thing, I like them al dente at this stage. While they are cooking sauté the diced peppers to soften them slightly though not fully cooked.
When the lentils are done as you like them add the tomato purée and more of the water to make the soup base then add all the rest of the ingredients. Heat the soup through so that it thickens up and season to taste.