Place cubed potatoes in a pot of salted water, bring to a boil, and cook for 15 to 20 minutes until tender.
In a separate large pot, heat olive oil over medium heat and sauté onion, carrots, and celery for 6 minutes.
Add minced garlic, thyme, and smoked paprika to the vegetables and cook for 1 minute until fragrant.
Stir in the lentils, vegetable broth, and diced tomatoes with their juices.
Bring the stew to a boil, then reduce heat to low, cover, and simmer for 30 to 35 minutes until lentils are soft.
Drain the cooked potatoes and return them to the pot.
Mash the potatoes with soy milk and nutritional yeast until smooth, seasoning with half of the salt and pepper.
Season the finished lentil stew with the remaining salt and pepper.
Serve the lentil stew layered directly over a portion of the mashed potatoes.