Follow these detailed steps to create a perfect Lentil and Veggie Shepherd’s Pie. The key is to build flavor in layers, starting with the vegetable base and allowing the filling to simmer and develop its rich taste.

Part 1: Prepare the Savory Lentil Filling

1. Sauté the Aromatics: In a large Dutch oven or a heavy-bottomed pot, heat the 2 tablespoons of olive oil over medium heat. Once the oil is shimmering, add the finely chopped onions, carrots, and celery (this combination is known as a mirepoix). Sauté for 8-10 minutes, stirring occasionally, until the vegetables have softened and the onions are translucent. This slow cooking process is crucial for developing a sweet, flavorful foundation.
2. Add Mushrooms and Garlic: Add the chopped cremini mushrooms to the pot. Continue to cook for another 5-7 minutes, until the mushrooms have released their liquid and started to brown. Browning the mushrooms is key to unlocking their deep umami flavor. Stir in the minced garlic and cook for just one minute more until fragrant. Be careful not to burn the garlic, as it can become bitter.
3. Incorporate Pastes and Herbs: Stir in the 2 tablespoons of tomato paste, the dried thyme, and the crushed dried rosemary. Cook for another minute, stirring constantly. This step “toasts” the tomato paste and herbs, which deepens their flavor and removes any raw taste.
4. Simmer the Lentils: Pour in the rinsed lentils, the undrained can of diced tomatoes, the 4 cups of vegetable broth, and the soy sauce (or tamari). Stir everything together well, scraping up any browned bits from the bottom of the pot.
5. Cook to Perfection: Bring the mixture to a boil, then reduce the heat to low, cover the pot, and let it simmer for 40-50 minutes. Stir every 10-15 minutes to prevent sticking. The filling is ready when the lentils are tender but not mushy, and most of the liquid has been absorbed, resulting in a thick, stew-like consistency.
6. Final Touches: Once the lentils are cooked, turn off the heat. Stir in the 1 cup of frozen peas. The residual heat will cook them perfectly while keeping their bright color and texture. Season generously with salt and black pepper to your taste. Don’t be shy with the seasoning; the filling should be robustly flavored. Set the filling aside while you prepare the topping.

Part 2: Create the  Creamy Sweet Potato Topping

1. Boil the Potatoes: While the lentil filling is simmering, place the peeled and chopped sweet potatoes in a large pot. Cover them with cold, salted water. Bring the water to a boil over high heat, then reduce the heat to medium and cook for 15-20 minutes, or until the potatoes are very tender when pierced with a fork.
2. Drain and Mash: Drain the sweet potatoes thoroughly in a colander and return them to the hot, empty pot. Let them sit for a minute to allow any excess moisture to steam off—this is the secret to a creamy, not watery, mash.
3. Add Richness and Flavor: Add the vegan butter (or olive oil), unsweetened almond milk, nutmeg, salt, and white pepper to the pot with the sweet potatoes.
4. Whip to Creaminess: Using a potato masher or an electric hand mixer, mash the sweet potatoes until they are smooth and creamy. Using an electric mixer will yield an exceptionally light and fluffy result. Taste and adjust the seasoning as needed. It should be flavorful on its own.

Part 3: Assemble and Bake the Shepherd’s  Pie

1. Preheat and Prepare: Preheat your oven to 400°F (200°C).
2. Layer the Pie: Spread the prepared lentil and vegetable filling evenly into the bottom of a 9×13 inch casserole dish or a similar-sized baking dish.
3. Top with Sweet Potato: Carefully spoon the creamy sweet potato mash over the lentil filling. Use the back of a spoon or an offset spatula to gently spread it into an even layer, sealing the filling in.
4. Create a Textured Top: For a beautiful, rustic look and crispy edges, use the tines of a fork to create patterns or ridges across the top of the sweet potato mash. These peaks and valleys will brown beautifully in the oven.
5. Bake to Golden Perfection: Place the casserole dish on a baking sheet to catch any potential drips. Bake for 25-30 minutes, or until the filling is bubbling around the edges and the sweet potato topping is lightly golden brown on top. For an extra-browned top, you can place it under the broiler for the last 1-2 minutes, but watch it carefully to prevent burning.
6. Rest and Serve: Remove the Shepherd’s Pie from the oven and let it rest for at least 10-15 minutes before serving. This crucial step allows the filling to set, making it much easier to slice and serve.