01 - Set the oven temperature to 400°F.
02 - Place cubed potatoes in a large pot, cover with cold salted water, and bring to a boil. Simmer for 15 to 20 minutes until tender when pierced with a fork.
03 - Heat olive oil in a large skillet over medium heat. Add chopped onion, diced carrots, and celery; cook for 6 to 8 minutes until softened.
04 - Incorporate minced garlic, tomato paste, dried thyme, rosemary, and smoked paprika. Stir continuously for 1 minute until fragrant.
05 - Add rinsed lentils, vegetable broth, soy sauce or tamari, salt, and pepper. Bring mixture to a simmer, cover, and cook for 25 to 30 minutes, stirring occasionally, until lentils are tender and most liquid is absorbed. Add frozen peas during the last 5 minutes.
06 - Drain cooked potatoes thoroughly and return to pot. Mash with plant-based or dairy milk, butter, salt, and pepper until smooth and creamy.
07 - Transfer lentil filling to a lightly greased 9x13-inch baking dish. Spread mashed potatoes evenly on top and create ridges with a fork to enhance crispness.
08 - Place assembled dish in oven and bake for 20 to 25 minutes until the top is golden and filling is bubbling. Optionally, broil on high for 2 to 3 minutes for a crispier surface.
09 - Allow the dish to rest for 5 minutes before serving.