Press the tofu in a tofu press or under a plate topped with something heavy. Let sit for 20 minutes then discard the water that has collected.
Soak the cashews in hot water for 15 minutes. Discard the soaking water.
Add the butter to a frying pan and bring the heat to medium.
Add the onion and garlic and cook for 8 minutes stirring frequently.
In a blender add the pressed tofu, drained cashews, broth, nutritional yeast, tahini, lemon juice, olive oil, miso paste, garlic granules, onion granules, 1 teaspoon salt, and ½ teaspoon pepper. Blend until smooth.
Preheat the oven to 400 F
Slice the potatoes into ⅛-inch slices by hand or with a mandolin.
Spread half of the potatoes into a 9x13-inch baking dish that has been sprayed with cooking spray.
Top with half of the creamy sauce and all of the onion mixture.
Add the rest of the potatoes and then top with the remaining sauce, and remaining salt and pepper. Bake for 55 minutes or until the potatoes are tender.