Preheat the oven to 375°F. Lightly grease a 9x13 baking dish with butter and set aside.
Cook the bacon pieces in a medium skillet over medium-low heat for 9-11 minutes or just until crispy. Be careful not to over crisp as they will continue to crisp slightly when baked. Using a slotted spoon, transfer the cooked bacon to a paper towel-lined plate to drain then set aside. While the bacon is cooking, proceed with cooking the potatoes.
Cover the cubed potatoes with 2-inches cold water in a large pot. Bring the water to a boil, once boiling reduce the heat to medium-high and cook for 10-12 minutes or until fork tender.
Drain the potatoes and return to the pot. Add the sour cream, heavy cream, unsalted butter, seasoned salt, garlic powder, and black pepper. Mash the potatoes until smooth and all the ingredients are fully incorporated.
Add ½ cup shredded cheddar and ½ cup shredded colby-jack cheese to the mashed potatoes and stir to combine.
Transfer the potato mixture to the prepared baking dish and smooth to an even layer.
Top the potatoes with the remaining 1 cup shredded cheddar and 1 cup shredded colby-jack cheeses.
Sprinkle the reserved cooked bacon over the cheese and bake for 20-25 minutes or just until the cheese is melted.
Remove from the oven and top with the chopped green onion for garnish before serving.
Notes
The combination of cheddar and colby-jack cheeses makes for a wonderful combination in this casserole due to the great flavor the cheddar adds, while colby-jack cheese melts really well. You can substitute all cheddar or all colby-jack in this recipe and it will still yield wonderful results.
Yukon gold potatoes are a great substitute for the russet potatoes as they also yield a creamy texture when mashed.
Chives can be substituted for the green onions as garnish. They have a slightly milder onion flavor than green onions.