Rinse the lentils thoroughly, then combine them with vegetable broth in a large pot. Bring to a boil over medium heat, then reduce to a simmer and cook for 25-30 minutes until tender but still firm. Drain any excess liquid and set aside.
While the lentils cook, peel and dice the potatoes. Place them in a pot of boiling water and cook for about 15 minutes until easily pierced with a fork. Drain and mash until smooth, then stir in vegan butter or olive oil, and season with salt and pepper. Cover and set aside.
Heat olive oil in a sauté pan over medium heat. Add chopped onions and diced carrots, cooking for 8-10 minutes until they soften and start to caramelize, filling your kitchen with a sweet aroma. Add minced garlic during the last minute and stir until fragrant.
Add the cooked lentils to the sautéed vegetables, stirring well. Toss in a bay leaf and thyme, then season with salt and pepper. Continue cooking for another 5 minutes to let flavors meld, stirring occasionally. Remove the bay leaf before assembling.
Preheat your oven to 200°C (400°F). Transfer the lentil and vegetable mixture into a baking dish, spreading it evenly.
Spread the fluffy mashed potatoes evenly over the lentil filling, smoothing with a spatula or the back of a spoon. For a golden crust, you can sprinkle a little nutritional yeast or vegan cheese on top.
Bake in the preheated oven for 25-30 minutes until the top is golden and crispy around the edges, with bubbling filling around the sides. You can broil for an extra 2-3 minutes for a crispier crust if desired.
Remove the pie from the oven and let it rest for 5-10 minutes. This allows the filling to set and makes serving easier. Garnish with chopped herbs if you like.
Scoop generous portions onto plates, ensuring each slice has crispy topping and flavorful filling. Serve hot with your favorite side for a cozy, satisfying meal.
