Preheat the oven to 375°F (190°C) and grease a 9x13 inch casserole dish.
In a large pot, cover the cubed potatoes with water, add a pinch of salt, and bring to a boil. Cook for 12-15 minutes until fork-tender. Drain, making sure no water is left.
In a large bowl, mash the boiled potatoes. Stir in the vegan sour cream, melted vegan butter, plant-based milk, garlic powder, onion powder, salt, and pepper. Mix until smooth and creamy.
In a small bowl, mix the panko breadcrumbs with 3 tablespoons of melted vegan butter until coated. Add grated cheddar cheese and mix to combine.
Spread the mashed potato mixture evenly into the greased casserole dish. Sprinkle the breadcrumb and cheese mixture evenly over the casserole.
Place the casserole in the oven and bake for 25-30 minutes, or until the top is golden brown and the casserole is heated through.
Remove from the oven, sprinkle with vegan bacon, and chopped onion. Let it cool for a few minutes before serving.
Notes
Refrigerator: Store leftover mashed potato casserole in an airtight container, or cover the casserole dish with plastic wrap and place it in the refrigerator for 3-4 days.
Freezer: For longer storage, freeze the casserole in a freezer-safe container for up to 3 months. Thaw overnight in the refrigerator before reheating.
Reheat: To reheat the leftovers, place them in an oven-safe dish and bake at 350°F until warmed through. Alternatively, you can heat individual servings in the microwave until warm.