Pour the olive oil into a large, deep pot set over medium-high heat. Stir in the onion and garlic; cook until transparent, about 5 minutes. Add the celery, carrots, vegetable broth, and water; cook for 10 to 15 minutes. Reduce heat to medium, pour in the tomatoes, and cook 10 to 15 minutes more. Reduce heat to low, and mix in the lentils and chestnuts; cook for 1 hour until the chestnuts soften. Pour in more water to thin the stew, if desired. Season to taste with salt and pepper.
