Preheat your oven to 400°F and lightly grease a 9×13-inch baking dish.: The oven heat creates steady, even baking so the potatoes become tender without drying out. You should feel the warmth when you open the oven door and hear the faint hum of the preheating element as it reaches temperature. Greasing the dish prevents sticking and encourages those golden edges to release easily. A common mistake is skipping preheating, which can lengthen the bake and produce uneven doneness; always wait until the oven signals it has reached 400°F .
Peel the potatoes and slice them into 1/8-inch rounds. Set aside in a bowl of cold water if not using immediately.: Thin, uniform slices are the secret to even cooking and a cohesive texture. As you slice, notice the smooth, pale flesh of the potatoes and the slight resistance of a well sharpened knife or mandoline. Placing slices in cold water prevents them from oxidizing and keeps them bright. Avoid irregular thickness, because thick slices may remain firm while thin ones overcook, producing an inconsistent casserole.
Place the cashews, vegetable broth, almond milk, nutritional yeast, and salt in a high-speed blender. Blend until very smooth to create a creamy sauce. If the sauce is too thick, thin it with a little bit more almond milk.: When you blend these, the motor should hum steadily and the mixture will first pulse then smooth into a glossy, pale sauce. The aroma will shift from raw nuts to a warm, savory scent as the nutritional yeast integrates. If the sauce seems too thick, thinning it with small amounts of extra almond milk helps it coat the slices evenly. A typical error is under blending, which leaves a grainy texture instead of the desired velvety cream.
Heat olive oil in a large skillet over medium heat. Add the sliced onion and sauté for 3-4 minutes until translucent. Stir in the minced garlic and cook for an additional 1 minute.: You will see the onion soften and become glossy, and the scent will turn sweet and aromatic. Stirring prevents uneven browning and helps the edges to caramelize gently. After the onion is translucent, stir in the minced garlic and cook for an additional 1 minute until fragrant; garlic releases a fragrant warmth that signals readiness. The mistake to avoid here is letting the garlic brown too much, which introduces a bitter note to the dish.
Spread half of the sliced potatoes evenly in the prepared baking dish. Pour half of the blended sauce over the potatoes, then distribute the sautéed onions and garlic evenly on top. Layer the remaining potato slices over this mixture, then pour the rest of the sauce over the casserole.: As you layer, notice how the sauce settles into the crevices, coating each slice and promising tenderness in the finished dish. The interplay of creamy sauce and thin potato layers ensures every forkful has balance. Press lightly so the layers make contact without compressing them flat; this allows even baking and sauce penetration. A common pitfall is uneven layering, which can trap sauce in pockets and leave other areas dry, so take a moment to distribute the components evenly.
Cover the dish with foil. Bake in the preheated oven until the potatoes are fork tender, about 60-70 minutes. Take the dish out of the oven, remove the foil, and let sit for 5 minutes.: During baking you will notice gentle bubbling at the edges and a faint golden hue forming where the sauce kisses the dish. The smell will deepen into savory comfort, and a fork should slide through the center slices without resistance when ready. Removing the foil toward the end encourages slight browning on top, creating contrast in texture. Letting the casserole rest for 5 minutes allows the sauce to settle and thicken, making it easier to slice. Avoid opening the oven repeatedly, which cools the cavity and extends cook time.
Optionally sprinkle chopped fresh chives over the top before serving while warm.: The final scatter of chopped chives adds a bright green contrast and a fresh oniony note that lifts the dish. When you add them warm, they release a gentle fragrance without wilting, preserving color and texture. This finishing touch changes the first impression from purely rich to balanced and inviting. A misstep is adding herbs too early, which causes them to lose vibrancy, so reserve them until right before serving.