Heat up olive oil in a pot. Add onion and stir for a minute. Add celery, carrots, ½ tsp salt and ½ tsp ground black pepper. Sauté, stirring occasionally for 3 minutes. Add garlic, stir and cook until soft, 5-7 minutes. Add ground cumin, stir and cook for a minute. Add tomato paste, stir and cook for a minute.
Add the rinsed lentils, water, 2 tsp salt and 1 tsp ground black pepper. Cover and bring to a full boil, about 7 minutes. Stir, turn down heat to low, cover the pot and simmer for about 40 minutes or until lentils are cooked. Cooking time depends on the lentils you are using. Stir the soup occasionally and add more boiled water if necessary.
When the lentils are cooked, taste and add more salt if you like. Add spinach leaves (if using). Stir until wilted and turn off heat. Add 1 tbsp of vinegar, stir, taste, and add more if you like. Serve hot with crusty bread.