Place diced potatoes in a pot of salted water. Bring to a boil and cook for 15-20 minutes until tender.
Drain and mash with plant-based milk, vegan butter, garlic powder, salt, and pepper. Set aside.
Heat olive oil in a large pan over medium heat. Add onion, garlic, carrots, and celery. Sauté for 5 minutes until softened.
Add mushrooms, thyme, rosemary, and smoked paprika. Cook for another 3 minutes.
Stir in tomato paste and soy sauce, mixing well.
Add cooked lentils and vegetable broth. Simmer for 10 minutes, allowing flavors to meld.
Stir in frozen peas, season with salt and pepper, and remove from heat.
Preheat oven to 375°F (190°C).
Transfer the lentil filling to a baking dish and spread evenly.
Top with the mashed potatoes, spreading evenly with a spatula.
Bake for 20 minutes until slightly golden on top. For extra crispiness, broil for the last 5 minutes.
Serve