If you’re looking for a casserole with an amazingly crunchy bread crumb topping, this is your new go-to recipe. Danielle of So Good Blog says “leftover mashed potatoes never had it so good,” and we agree.
Preheat the oven to 350 degrees. Butter a small (8-inch) casserole dish.
Lay the bacon strips out in a large, cold skillet, and then place the skillet over medium heat. Cook the bacon until it becomes crispy, about 6 minutes, then transfer to a plate to cool.
Pour all but 1 tablespoon of bacon fat from the skillet and then add the sliced onions and peppers along with some salt and pepper. Cook until the vegetables become tender, about 6 minutes. Transfer the softened vegetables to the plate along with the bacon to cool.
Spread the mashed potatoes in the bottom of the prepared casserole dish. Top with the sour cream and spread into an even layer. Crumble in the bacon followed by the softened vegetables. Shingle the sliced boiled potato over the vegetables, sprinkle with salt and panko, and dot with the cubes of butter. Finish with a layer of shredded cheese and sprinkle with ground black pepper. Bake the potato casserole until the cheese melts and becomes slightly browned, about 30 minutes. Allow the casserole to cool for 5 minutes, then dig in!