In a large pot heat the olive oil over medium heat. Add the diced onion and the garlic. Sauté for 1-2 min.
Add the carrots and celery and sauté for 3-5 minutes.
Stir and add the lentils, smashed tomatoes (you can also smash the tomatoes with your spoon after you add them in) and vegetable stock. Bring to a boil for 1-2 min.
Lower the heat to the minimum. Cover the pot and simmer for 60-90 minutes. Cooking time depends on the kind of lentils you are using. I am using green lentils that require longer cooking. Check lentils often until they have the desired texture. Adjust liquids too, you might need to add a little water or stock to achieve your desired soup consistency).
Add salt and freshly ground pepper to taste.
Serve lentils in bowls with a slight sprinkle of red hot chili pepper flakes, a 1 tbsp olive, a tbs of red wine vinegar (optional) and warm crusty bread on the side.
Notes
I use authentic San Marzano D.O.P. tomatoes; I love the sweet taste and velvety texture.
Instead of green lentils, you can use brown lentils. Just adjust the cooking time. Brown lentils need approximately 35-45 minutes to cook.