Preheat oven to 400°F for 15-20 minutes until the top becomes golden.
Place chopped sweet potatoes and butternut squash in a large pot and fill with water until they are just covered. Bring to a boil over high heat. Then reduce to medium and cook for 15-20 minutes or until they slide off a knife very easily when pierced.
While the sweet potatoes and butternut squash are cooking, prepare the filling.
Place a pan on medium heat and add the oil. When the oil shimmers add the onions, garlic and leeks and cook until soft and translucent.
Add in the spices, mushrooms and heat until the mushrooms soften (5-6 minutes).
Mix in the mashed tofu, walnuts, herbs and cook until all combined and fragrant.
Pour this mixture evenly into the bottom of a greased baking dish and set aside.
For the topping, drain and transfer the sweet potatoes and butternut squash into a large mixing bowl. Use a masher or fork to mash until smooth.
Puree the soaked cashews in enough water to make a soft and smooth paste (2-3 tablespoons).
Combine the cashew puree with the mashed topping. Add salt and pepper to taste. I also added some fresh thyme into the topping. Mix until smooth and all combined.
Layer the sliced zucchini (if using) onto the tofu filling and then spoon on the topping. Smooth down with a fork or spoon.
Place the baking dish into a preheated oven and bake for 20 minutes or until the potatoes are lightly browned on top and the edges are bubbling.
At this time you can set the oven on broil and let the top brown for a crispier topping but watch is closely so it doesn’t burn, 2-3 minutes is all it should take.
Remove it from the oven and let it set for 5 minutes before serving.