First, make the mashed potatoes. Bring a large pot of water to boil. While it's warming, peel the sweet potatoes and cut them into large cubes (about 1 1/2 inches). When the water is ready, add the sweet potatoes and boil for 15-20 minutes, or until they're very tender when pierced with a fork.
Drain the potatoes and add them to a large mixing bowl. Mash roughly with a handheld potato masher, and then add the salt, garlic powder, almond milk, and oil. Mash well, but leave a little texture. Season to taste and set the potatoes aside. You can do this up to three days in advance, storing the mashed potatoes in an airtight container in the fridge until you're ready to use them.
Preheat the oven to 350F and lightly oil a 9x13 casserole dish.
To cook the lentils, place them in a medium or large saucepan and cover them with 3 inches of water. Bring the water to a boil and reduce it to a simmer, then cover. Simmer the lentils for 20-25 minutes, or until they're tender but still have some chew; I recommend checking them at the 18 minute mark to be sure they don't get mushy. When the lentils are ready, drain them and set them aside. (This can also be done up to 2-3 days in advance.)
Heat the olive or grapeseed oil in a very large skillet or saute pan over medium high heat. Add the onions and shallots, along with a pinch of salt. Cook the onions for 5 minutes, or until they're becoming clear. Add the garlic and cook for another minute, or until it's very fragrant. Add the carrots and celery. Cook for 2-3 minutes, stirring often, and then add the mushrooms, thyme, rosemary, sage, and vinegar. Give the mushrooms another generous pinch of salt, and then cook for 7-8 minutes, or until the mushrooms have released all of their juices. Stir in the wine, if using, and allow it to cook off.
Add the cooked lentils and broth to the pot. Heat everything and mix it together. Check the lentil mixture for seasoning, and add salt and pepper as needed (I add about a 1/2 teaspoon salt and about 1/4 teaspoon pepper).
Remove the lentil/mushroom mixture from heat, and then pour it into your prepared casserole dish. Top it with the sweet potatoes, and then gently use a spatula to spread them out over the lentil mixture. If desired, top it all with 1/3 cup bread crumbs.
Bake the shepherd's pie for 40-45 minutes, or until it's crispy on top. Serve.
Leftover pie will keep for up to three days in an airtight container in the fridge.