Prep: Cook the lentils, if you haven’t already. Peel and chop your potatoes first, then prepare the rest of the ingredients. Set a 9x13” casserole dish aside. Preheat the oven to 400F.
Cook the Potatoes: Add the potatoes to a large pot or Dutch oven and cover with water. Bring to a boil over high heat, then reduce to medium-high heat and boil for 10 to 15 minutes, until fork-tender. Remove the pot from the heat, drain the potatoes well, and return to the pot to cool.
Prep the Filling: In the meantime, warm the oil in a large sauté pan over medium heat. Add the onion and sauté onion 3 minutes, until translucent. Then, add in the tomato paste and stir it into the onion. Fry for an additional 3 minutes. Add in the mushrooms, rosemary, thyme, parsley, black pepper, and salt. Sauté for 3 to 5 minutes, until the mushrooms have reduced in size. Add the garlic, vegan Worcestershire (optional), and flour, then sauté for an additional minute. Finally, add in the lentils, vegetables, and broth. Mix well and bring to a low simmer, then remove from the heat and set aside.
Mash the Potatoes: Add 3 tablespoons of the vegan butter, milk, nutritional yeast, garlic powder, salt, and black pepper (to taste) to the potatoes in the large pot. Mash well with a potato masher, then use a spoon or spatula to mix well, until you get a creamy uniform texture.
Assemble: Transfer the lentil mixture to the base of the casserole dish, then dollop spoonfuls of mashed potatoes evenly over the mixture. Use a spatula to spread the potatoes evenly. Then, use a fork to create a wavy or crosshatched textured pattern over the top of the potatoes - this helps then brown more. Melt the remaining tablespoon of butter, then brush it over the top of the potatoes.
Bake: If the casserole dish is full, place it on top of a baking dish when putting it in the oven, so it does not spill onto the oven base. Bake in the middle rack of the oven for 27 to 30 minutes, until bubbly. Transfer the casserole to the top rack of the oven and Broil for 5 to 10 minutes, with the door slightly ajar, for a crispier finish. Let the shepherd’s pie cool (and set) for 15 minutes before serving. Serve warm; leftovers can be store in the refrigerator for up to 5 days, and reheated in the oven, toaster oven, or microwave.