Prepare and Cook the Potatoes: Cut large potatoes in half to ensure even cooking. Place them in a large pot and cover with water. Bring to a gentle boil over medium-high heat. Salt the water generously, then reduce the heat slightly, cover the pot, and simmer the potatoes for 20–30 minutes or until fork-tender and easy to mash.
Mash and Season the Potatoes: Drain the cooked potatoes thoroughly, then return them to the hot pot briefly to let excess moisture evaporate. Transfer to a large mixing bowl and mash using a masher or fork until smooth. Mix in the vegan butter and season with salt and pepper to taste. Cover loosely and set aside.
Preheat Oven and Prepare Baking Dish: Set your oven to 425°F (218°C). Lightly grease a 2-quart baking dish (a 9×13-inch pan works well too). Place a baking sheet under the dish during baking to catch any spills.
Sauté Aromatics for Filling: In a large saucepan, heat olive oil over medium heat. Add the diced onion and cook until softened and lightly browned, about 5 minutes. Stir in the minced garlic and cook for 1 more minute, being careful not to burn it.
Simmer the Lentils and Seasonings: Mix in the tomato paste (if using), along with a pinch of salt and pepper. Add the lentils, vegetable broth, and thyme. Stir to combine. Bring to a low boil, then cover and simmer on low heat for 35–40 minutes, or until lentils are soft and cooked through.
Finish and Thicken the Filling: Once lentils are tender, remove the lid and simmer uncovered for another 5–10 minutes, stirring frequently to allow excess liquid to cook off. Add the frozen mixed vegetables during the last 10 minutes of simmering, stirring occasionally so everything melds together.
Optional Thicken If Desired: To achieve a thicker texture, stir in 2–3 tablespoons of the prepared mashed potatoes. Alternatively, blend ½ cup of the lentil mixture with 2 tablespoons of cornstarch or arrowroot powder and stir it back into the pan to thicken.
Assemble the Pie: Spoon the lentil and vegetable mixture into the prepared baking dish and spread it into an even layer. Carefully top with the mashed potatoes, smoothing the surface with the back of a spoon or a spatula. Sprinkle a bit more sea salt and cracked pepper on top.
Bake Until Lightly Golden: Place the dish (on top of a baking sheet) in the oven and bake for 10–15 minutes, or until the top of the potatoes develops a light golden color. Let it cool for 5–10 minutes before serving. The filling will thicken further as it rests.