1. 1. Boil Potatoes: Place the peeled and quartered potatoes in a large pot and cover with cold, salted water. Bring to a boil over high heat, then reduce heat to medium and simmer for 15-20 minutes, or until the potatoes are very tender and easily pierced with a fork.

1. 1. Drain and Mash: Drain the potatoes thoroughly in a colander. Return them to the hot, empty pot and place it back on the stove over low heat for 1-2 minutes, shaking the pot occasionally. This helps to dry out any excess moisture, leading to fluffier mash.

1. 1. Add Enrichments: Remove the pot from the heat. Add the cubed butter and allow it to melt over the hot potatoes. Add the warmed milk, 1 cup of shredded cheddar cheese, and the grated Parmesan cheese.

1. 1. Mash to Perfection: Mash the potatoes until smooth and creamy. You can use a potato masher for a slightly rustic texture or a potato ricer for an ultra-smooth finish. Be careful not to over-mash, as this can make them gummy.

1. 1. Season: Season generously with salt, freshly ground pepper, and a pinch of nutmeg (if using). Taste and adjust seasonings as needed. Set aside and keep warm, or cover to prevent a skin from forming.

Part 2: Making the Rich Vegetarian Filling

1. 1. Sauté Aromatics: Heat the olive oil in a large oven-safe skillet or Dutch oven over medium heat. Add the chopped onion, carrots, and celery. Cook, stirring occasionally, for 8-10 minutes, until softened and the onions are translucent.

1. 1. Cook Mushrooms: Add the finely chopped mushrooms to the skillet. Cook for 5-7 minutes, stirring occasionally, until they have released their moisture and started to brown. This step is crucial for developing their umami flavor.

1. 1. Add Garlic and Tomato Paste: Stir in the minced garlic and cook for 1 minute until fragrant. Add the tomato paste and cook for another 1-2 minutes, stirring constantly, allowing it to caramelize slightly.

1. 1. Incorporate Lentils and Herbs: Add the rinsed lentils, dried thyme, crushed rosemary, and bay leaf to the skillet. Stir to combine everything well with the vegetables.

1. 1. Simmer the Filling: Pour in the vegetable broth, soy sauce (or tamari), and vegetarian Worcestershire sauce. Bring the mixture to a gentle simmer. Reduce the heat to low, cover, and cook for 30-40 minutes, or until the lentils are tender but still hold their shape. Stir occasionally to prevent sticking.

1. 1. Season and Thicken (if needed): Once lentils are tender, remove the bay leaf. Taste the filling and season with salt and pepper as needed. If the gravy seems too thin for your liking:

      • • Flour method: Sprinkle 1-2 tablespoons of flour over the filling and stir well. Cook for another 2-3 minutes, stirring, until the gravy thickens.
      • • Cornstarch slurry: Mix 1 tablespoon of cornstarch with 2 tablespoons of cold water until smooth. Stir the slurry into the simmering filling and cook for 1-2 minutes, until thickened.

1. 1. Add Frozen Vegetables: Stir in the frozen peas and corn (if using). Cook for another 2-3 minutes, just until they are heated through.

Part 3: Assembling and Baking the Shepherd’s Pie

1. 1. Preheat Oven: Preheat your oven to 400°F (200°C).

1. 1. Layer the Pie:

      • • If your skillet is not oven-safe, transfer the hot lentil filling to a 9×13 inch baking dish or a similar-sized casserole dish, spreading it into an even layer.
      • • Carefully spoon the prepared cheesy mashed potato topping over the lentil filling. Spread it gently with a spatula to cover the filling completely, sealing the edges if possible to prevent the filling from bubbling over too much.
      • • For a decorative touch, you can use the back of a spoon to create swirls or a fork to create ridges on the surface of the mash. These will crisp up nicely in the oven.

1. 1. Add More Cheese: Sprinkle the remaining 1/2 cup of shredded cheddar cheese evenly over the mashed potato topping.

1. 1. Bake: Place the Shepherd’s Pie in the preheated oven. Bake for 20-25 minutes, or until the filling is bubbly around the edges and the cheesy mash topping is golden brown and slightly crispy in spots. If the top starts to brown too quickly, you can loosely tent it with aluminum foil.

1. 1. Rest Before Serving: Once baked, remove the Shepherd’s Pie from the oven and let it rest for at least 10-15 minutes before serving. This allows the filling to set slightly and the molten cheese to cool a bit, making it easier to serve and preventing burnt tongues.

1. 1. Garnish and Serve: Garnish with freshly chopped parsley or chives, if desired, before serving.