In a medium sauce pot, add your lentils, quinoa and veggie broth and bring to a boil. Reduce heat, and let simmer uncovered for 20 minutes stirring occasionally, until liquids are mostly absorbed. (Towards the end of cooking, stir more often.) Set aside.
Meanwhile, add your diced sweet potato to a small pot, covered with water, bring to a boil and cook until fork tender, approx. 8 minutes. Drain and set aside.
To a large skillet, add the leek, zucchini, mushrooms, garlic powder, thyme, and salt and sauté over medium-high heat until softened, approx 5 minutes. Stir often to prevent sticking. You should not need any water or oil since the mushrooms will release moisture as they cook.
Add the cooked quinoa and lentils, plus the sautéed veggies to a 9"x13" (or similar) baking dish and mix well.
To make the cheese sauce, add your cooked sweet potato, and all remaining cheese sauce ingredients to your blender and process until smooth. Evenly pour the sauce over your lentil, quinoa, and veggie mixture, and sprinkle with more thyme, if you'd like.
Cover the dish with parchment paper and then aluminum foil and bake for 15 minutes. Then remove the foil/paper and bake another 5 minutes to slightly brown the top. Serve hot.
Notes
Cook the quinoa and lentils in a delicious broth (instead of water) for added depth of flavour.Prep and assemble the casserole in advance if possible. Then you can have dinner on the table in 20 minutes on those busy weeknights.This vegan quinoa casserole tastes best when freshly baked and the cheese sauce is hot and melty. As it cools down to room temperature, the cheese layer will begin to harden. To return it to a melty state, just pop it back in the oven and reheat it.Leftovers keep for up to 4 days in the refrigerator. Just reheat and enjoy.