Preheat oven to 350 degrees F (175 degrees C). Lightly grease a 9x13-inch baking dish.
Place potatoes into a large pot and cover with salted water; bring to a boil. Reduce heat to medium-low and simmer until tender, about 20 minutes.
Drain and return potatoes to pot.
Meanwhile, arrange bacon in a large skillet and cook over medium-high heat, turning occasionally, until evenly browned, about 10 minutes.
Drain bacon on paper towels and crumble.
Combine 1 1/2 cup Cheddar cheese, evaporated milk, sour cream, salt, and black pepper in the pot with potatoes. Mash with a potato masher until creamy.
Spread potato mixture into prepared baking dish.
Bake in preheated oven for 25 minutes; sprinkle with bacon, green onions, and remaining 1/2 cup Cheddar cheese. Return to oven and continue baking until cheese is melted, about 5 minutes more.
