Part 1: Prepare the Mashed Potato Topping

1. Boil the Potatoes: Place the peeled and chunked potatoes in a large pot. Cover them with cold water by about an inch. Add 1 teaspoon of salt to the water. Bring the pot to a boil over high heat, then reduce the heat to medium and simmer gently for 15-20 minutes, or until the potatoes are very tender when pierced with a fork. They should offer almost no resistance.
2. Drain Thoroughly: Once tender, drain the potatoes extremely well in a colander. Let them sit for a minute or two to allow excess steam and moisture to escape – this is crucial for fluffy mash. Wet potatoes lead to gummy mash.
3. Mash the Potatoes: Return the drained potatoes to the warm pot (off the heat) or transfer them to a large bowl. Add the softened butter (or vegan butter), warmed milk (or plant-based milk), ½ teaspoon salt, ¼ teaspoon pepper, and a pinch of nutmeg (if using). You can also add nutritional yeast now if aiming for a cheesy flavour.
4. Achieve Smoothness: Mash the potatoes until smooth and creamy using a potato masher, ricer (for the smoothest texture), or even a sturdy fork. Avoid over-mashing, especially if using an electric mixer, as this can make them gluey. Taste and adjust seasoning if necessary. Set the mashed potatoes aside while you prepare the filling. Keep them covered to stay warm.

Part 2: Create the Rich Lentil and Vegetable Filling

1. Sauté the Aromatics: Heat the 2 tablespoons of olive oil in a large, deep skillet or Dutch oven over medium heat. Once shimmering, add the finely chopped onion, carrots, and celery (this combination is known as a mirepoix). Cook, stirring occasionally, for 8-10 minutes, until the vegetables have softened and the onion is translucent. This slow cooking builds a deep flavour base.
2. Add Garlic and Mushrooms: Stir in the minced garlic and chopped mushrooms (if using). Cook for another 2-3 minutes, until the garlic is fragrant and the mushrooms have released some of their moisture and started to brown slightly.
3. Incorporate Tomato Paste and Lentils: Add the tomato paste and stir constantly for 1 minute. Cooking the tomato paste briefly intensifies its flavour. Add the rinsed green or brown lentils and stir to coat them with the vegetable mixture.
4. Simmer the Filling: Pour in the vegetable broth. Add the soy sauce (or tamari), vegan Worcestershire sauce, dried thyme, dried rosemary, and the bay leaf. Stir everything together well.
5. Bring to a Boil and Simmer: Increase the heat and bring the mixture to a boil. Once boiling, reduce the heat to low, cover the skillet or Dutch oven, and let it simmer gently for 30-40 minutes. Stir occasionally to prevent sticking. The lentils should be tender but still hold their shape, and most of the liquid should be absorbed. If the mixture looks too dry during simmering, add a splash more broth or water.
6. Thicken the Filling: Remove the bay leaf. In a small bowl, whisk together the cornstarch (or flour) and 3 tablespoons of cold water until smooth, creating a slurry. Pour this slurry into the simmering lentil mixture, stirring constantly. Continue to cook for another 2-3 minutes, stirring, until the filling has thickened to a nice gravy-like consistency. It should be thick enough to hold its shape but not dry.
7. Stir in Peas and Parsley: Stir in the frozen peas and cook for just 1-2 minutes more, until they are heated through but still bright green. Turn off the heat. Stir in the chopped fresh parsley (reserving a little for garnish if desired).
8. Taste and Adjust: Taste the filling carefully and adjust seasoning with more salt and pepper if needed. Remember the potato topping will be milder, so ensure the filling is well-seasoned.

Part 3: Assemble and Bake the Shepherd’s Pie

1. Preheat Oven: Preheat your oven to 400°F (200°C).
2. Transfer Filling: Pour the hot lentil and vegetable filling into a suitable oven-safe baking dish (a 9×13 inch dish or a similar sized deep casserole dish works well). Spread it out evenly.
3. Top with Mashed Potatoes: Carefully spoon the prepared mashed potatoes over the lentil filling. Start by dolloping spoonfuls around the edges to create a seal, then fill in the center. Gently spread the potatoes evenly over the filling, being careful not to mix the layers.
4. Create Texture (Optional): Use the back of a fork or a spoon to create swirls, peaks, or ridges in the potato topping. These raised areas will catch the heat and brown nicely, adding visual appeal and crispy texture.
5. Add Cheese (Optional): If desired, sprinkle shredded cheese (like cheddar or Gruyere) or more nutritional yeast over the potato topping for extra flavour and browning.
6. Bake: Place the baking dish on a baking sheet (to catch any potential spills) and transfer it to the preheated oven. Bake for 20-25 minutes, or until the filling is bubbling around the edges and the potato topping is heated through and nicely golden brown on top.
7. Broil for Extra Browning (Optional): For a deeper golden-brown, crispier topping, you can switch the oven to the broil setting for the last 2-3 minutes. Watch it VERY carefully during this stage, as the topping can burn quickly under the broiler. Remove as soon as it reaches your desired colour.
8. Rest Before Serving: Once out of the oven, let the Vegetarian Shepherd’s Pie rest for 10-15 minutes before serving. This allows the filling to set slightly, making it easier to serve neat portions.
9. Garnish and Serve: Garnish with the remaining fresh parsley, if desired. Serve hot and enjoy!