Spray 9x11 glass or metal baking pan with Pam. Spread potatoes in pan.
Chop scallions and scatter on top of potatoes.
Zest 1 lemon. Juice both lemons. Pour lemon juice over potatoes.
Sprinkle zest, salt, and pepper on potatoes.
Top with butter pieces.
At this point, potatoes can be wrapped tightly in plastic wrap and refrigerated.
Remove plastic wrap. Put pan in preheated 400° oven for 1 ½ hours, stirring occasionally.
If potatoes brown before they are tender, cover loosely with foil until done.