Fry the vegetables. Heat the vegetable oil in a deep frying pan or a heavy-bottomed saucepan. Add the chopped bell pepper, onion and carrot to fry. Cook for about 2-3 minutes, stirring occasionally.
Prepare the remaining vegetables. Add tomatoes, garlic, salt and a pinch of sugar to the pan with the vegetables. Mix everything and leave it under the lid on low heat for another five minutes.
Cook the lentils. Add water and lentils to the pan with the vegetables and cook on high heat. Once boiling, reduce heat to low, cover the pan with a lid and cook the lentils for about 30 minutes, stirring occasionally.