Preheat the oven to 400 degrees F (200 degrees C). Grease a 2-quart casserole dish.

Bring a large pot of salted water to a boil. Add potatoes; cook until tender, about 15 minutes. Drain and mash.

Combine mashed potatoes, spinach, sour cream, butter, green onions, salt, and black pepper in a large bowl; spoon into the prepared dish.

Bake in the preheated oven for 15 minutes. Top with Cheddar cheese; continue baking until cheese is melted, about 5 minutes more.