Place the peeled and cubed potatoes in a large pot and cover with cold water, adding a generous pinch of salt. Bring to a boil over high heat, then reduce to a simmer and cook for 15-20 minutes until the pieces are fork-tender but not falling apart. Drain them well in a colander to remove excess water, which helps keep the mash fluffy rather than watery.

Return the drained potatoes to the warm pot and add the butter, warmed milk, sour cream, salt, pepper, and garlic powder if using. Use a potato masher to break them down, working in a gentle motion to incorporate the dairy until smooth with a few small lumps for texture. Avoid over-mashing to prevent a gummy consistency, and taste to adjust seasoning as needed.

Transfer the mashed potatoes to a greased baking dish, spreading them evenly with a spatula. Sprinkle the shredded cheddar cheese over the top, followed by the crumbled bacon for even distribution. This layering ensures every bite gets a mix of creamy mash and crunchy, cheesy elements without overwhelming the base.

Preheat your oven to 375°F and place the dish inside for 10-12 minutes, just until the cheese melts and bubbles slightly around the edges. Remove from the oven and let it cool for a couple of minutes to set. Scatter the sliced green onions on top right before serving to add a fresh, mild onion kick that brightens the rich flavors.