Boil sweet potatoes for at least 15–20 minutes, or until soft enough to puree.
Add carrots, onions, and celery to a large skillet over medium heat with a tablespoon of butter and allow them to soften.
Once the vegetables are softened, add the prepared lentils to the pan. Allow these to cook for several minutes with the other vegetables.
Add diced tomatoes with juices along with basil, spinach, and coconut aminos or tamari. Let this simmer for 10–15 minutes so the flavors to mingle.
When the sweet potatoes are soft all the way through, remove them from the heat and drain the water. Mash with a little salt and coconut milk until the consistency is perfect.
Add the lentil filling to a 9 x 13 pan and top with a layer of sweet potatoes. Bake in the oven for about 20 minutes until the topping becomes slightly browned. Allow to cool before serving.