In a large pot, heat olive oil over medium heat. Sauté diced onion, sliced carrots, and celery until soft.
Stir in minced garlic and cook for 1 minute. Add lentils, diced tomatoes, vegetable broth, thyme, bay leaf, salt, and pepper.
Bring to a boil then simmer for 25-30 minutes until lentils are tender.
Meanwhile, boil chopped potatoes in salted water until tender. Drain and mash with butter and milk until smooth.
Serve a generous scoop of mashed potatoes topped with lentil stew.
