Preheat the oven: Heat the oven to 350°F (175°C) and lightly grease a 9×13-inch baking dish.
Prepare the potatoes: Fully thaw the frozen potatoes and drain them well. Press out excess moisture with paper towels or a clean kitchen towel. This helps prevent watery dairy free funeral potatoes.
Cook the onion: Heat the olive oil in a skillet over medium heat. Add the chopped onion and cook for about 4–5 minutes, until softened and lightly golden.
Add the garlic: Stir in the minced garlic and cook for about 30–60 seconds, just until fragrant. Avoid browning it too much.
Blend the sauce: Add the plant milk, soaked cashews, nutritional yeast, miso, Dijon mustard, lemon juice, apple cider vinegar, cornstarch, garlic powder, onion powder, paprika, and a little salt to a high-speed blender.
Blend until smooth: Blend until the sauce is completely creamy with no noticeable pieces of cashew remaining.
Combine the potatoes: Add the thawed potatoes to a large mixing bowl. Fold in the cooked onion and garlic mixture.
Add the sauce: Pour the blended sauce over the potatoes and gently mix until the potatoes are evenly coated without becoming mashed.
Transfer to the dish: Spread the potato mixture evenly into the prepared baking dish.
Bake the casserole: Bake uncovered for about 30 minutes, allowing the potatoes to heat through and the sauce to begin thickening.
Prepare the topping: Mix the panko breadcrumbs, nutritional yeast, olive oil or melted vegan butter, salt, and black pepper in a small bowl.
Add the topping: Sprinkle the mixture evenly over the casserole after the first 30 minutes of baking.
Finish baking: Return the dish to the oven and bake for another 20–25 minutes, until the edges are bubbling and the topping is golden and crisp.
Rest before serving: Remove the casserole from the oven and let it rest for about 10 minutes. This allows the creamy sauce to settle and thicken.
