Stage 1 - Prep the Potatoes and Preheat
Peel your potatoes and slice them into ⅛-inch thick rounds consistency here genuinely matters, since uneven slices cook at different rates and you'll end up with some pieces tender and others still firm. Preheat your oven to 400°F and lightly grease your 9x13-inch baking dish. Set aside.
Stage 2 - Make the Creamy Sauce
Drain the soaked cashews. Add them to your high-powered blender along with the vegetable broth, non-dairy milk, nutritional yeast, and salt. Blend on high for a few minutes, until completely smooth with no grainy cashew texture remaining. This step is worth taking seriously an under-blended sauce leaves a slightly gritty mouthfeel that undermines the whole "creamy" promise of the dish.
Stage 3 - Sauté the Onion and Garlic
Heat the olive oil in your large sauté pan over medium heat. Add the sliced onion and sauté for 3–4 minutes, until translucent. Add the garlic and cook, stirring constantly, for another minute garlic burns quickly, so don't walk away during this step. Turn off the heat and set aside.
Stage 4 - Layer and Bake
Spread half of the sliced potatoes in an even layer across your prepared baking dish. Top evenly with half of the creamy sauce, then scatter the onion-garlic mixture across the top of that layer.
Layer the remaining potatoes on top, followed by the remaining sauce, spreading it evenly to cover everything.
Bake for 60 minutes, then check a few potatoes in the center for fork-tenderness. If they're not quite there yet, return to the oven for 10–20 more minutes, until fully cooked through. The sauce should look bubbly and thick by the time it's done.
Sprinkle vegan parmesan and fresh chives on top if desired, and serve.