Preheat oven to 350°F. Place potatoes in a large pot. Add cold water to completely cover the potatoes. Bring to a boil over high heat; boil until the potatoes are fork-tender, 15 to 20 minutes. Remove from heat and drain.
Transfer the potatoes to a large bowl. Using a potato masher, mash until fluffy. Add ½ cup Cheddar, cream cheese, ¼ cup scallions, milk, 2 tablespoons sour cream, egg, garlic-and-herb seasoning, salt and pepper; using a rubber spatula, gently mix until well combined (make sure not to overmix or the potatoes will become gluey). Transfer the mixture to an 8-inch-square baking dish. Sprinkle with the remaining ½ cup Cheddar.
Bake until the cheese is melted and the casserole is slightly browned around the edges, 25 to 30 minutes.
Meanwhile, combine 2 tablespoons water with the remaining 2 tablespoons sour cream in a small bowl, mixing to form a loose crema.
Drizzle the casserole with the crema; sprinkle with bacon and the remaining scallions. Additional reporting by Carrie Myers and Jan Valdez
