Add the sweet potatoes to a pot and top with water. Bring to a rolling boil then cover and let cook for 15 minutes.
Meanwhile, add the lentils and quinoa to a small saucepan. Cover with 2 cups of broth, the fresh thyme and bay leaves. Bring to a boil, cover and reduce to simmer. Cook for about 30 minutes until the water has been absorbed.
When the potatoes are done cooking, drain the water and add in non-dairy milk, butter, salt and pepper. With an electric mixer, whip potatoes until smooth and creamy. Cover and set aside.
When the lentils are done, preheat the oven to 425ºF and heat the olive oil in a large skillet (or dutch oven) over medium heat.
Add the mushrooms, onion, celery, carrot and garlic powder to the hot skillet and season with salt and pepper. Cook until veggies are tender, about 10 minutes. Add lentil-quinoa mixture and stir to combine. Stir in balsamic vinegar and 1 cup of broth.
Whisk the remaining 1/2 cup broth together with the arrowroot powder and in a separate dish. Once smooth, stir into the filling mixture.
Transfer the lentil mixture to a baking dish (I used a 2 qt, but 9x13 will also work). Smooth with a spatula and top with whipped potatoes. Spread the potatoes out in an even layer and smooth over with a spatula. Sprinkle with salt and pepper and bake for 15 - 20 minutes until potatoes have browned slightly.