Cook the Lentils: In a pot, combine 1/2 cup of dry lentils with 1 1/2 cups of water. Bring to a boil, then reduce heat and simmer for about 20 minutes, or until tender. Drain any excess liquid and set aside.
Prepare the Filling: In a skillet over medium heat, sauté the diced onion, carrots, celery, and mushrooms until softened, about 5-7 minutes. Add the minced garlic and cook for another minute until fragrant. Stir in the cooked lentils, frozen peas, tomato paste, dried thyme, rosemary, salt, and pepper.
Cook for an additional 5 minutes to allow the flavors to meld.
Make the Sweet Potato Topping: In a pot, cover the peeled and cubed sweet potatoes with water. Bring to a boil and cook until tender, about 15 minutes. Drain the sweet potatoes and mash them with the unsweetened almond milk, salt, and pepper until smooth and creamy.
Assemble the Pie: Transfer the lentil and vegetable filling to a baking dish (8 x 8) or deep pie dish. Spread the mashed sweet potatoes evenly over the top.
Bake: Preheat your oven to 375°F (190°C). Bake the shepherd's pie for about 20-25 minutes, or until the sweet potato topping is lightly browned.
Serve: Remove the pie from the oven and let it cool for a few minutes before serving.