Spray a 9x13-inch baking dish with nonstick cooking spray. Set aside.

Finely mince the scallions and set aside.

Dice the potatoes into 1/2-inch cubes and place in a large pot. Fill with enough water to cover the potatoes. Set the pot over medium-high heat and bring to a boil. Boil the potatoes until they can be easily pierced with a fork, 15-18 minutes.

Preheat the oven to 350°F.

Drain the potatoes into a colander, then transfer to a large mixing bowl. Add the minced scallion, crumbled bacon, butter, sour cream, milk, garlic powder, salt, black pepper, and shredded cheese to the bowl.

Use a potato masher or large fork to smash the potato mixture until it is mostly smooth, leaving some pea-sized pieces of potato. Add more milk if the mixture looks dry.

Transfer the mashed potato mixture to the casserole dish and use a rubber spatula to smooth it into an even layer.

Bake the potato casserole on middle rack of oven until it is bubbling at the edges and lightly golden brown on top, 20-25 minutes.

Check to see that casserole is done. Remove from oven or add time as needed.

Allow the casserole to rest for 5-10 minutes before serving.