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 and bring to a boil and cook until fork tender, approx minutes. Drain and set aside once cooked.
To a large skillet, add the leek, zucchini, mushrooms, garlic powder, thyme and salt and sauté 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 along with yout 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 using.
Cover the dish with parchment paper and then aluminum foil and bake for 15 minutes. Then remove the foil and bake an additional 5 minutes to slightly brown the top. Serve immediately.
This dish tastes best when fresh out of the oven when the cheese sauce is nice and melty. As it cools down to room temperature, the top layer will start to harden. To return it to a melty state, just pop it back in the oven and reheat.Leftovers keep for up to 5 days in the refrigerator. Just reheat and enjoy.