LAY 2 sheets of filo dough in prepared baking pan. BRUSH lightly with butter. TOP with 2 sheets of filo dough. BRUSH lightly with butter.
Continue layering 2 sheets and brushing with butter until you have 8-10 layers. BRUSH top with butter. Be sure dough reaches all sides of pan with every other layer (each sheet does not reach). If dough breaks, just layer put together the best you can and continue layering and buttering.
SPREAD spinach and cheese mixture into pan. BRUSH with butter and then LAYER remaining sheets of dough, 2 layers at a time, brushing with butter every 2 layers.
BAKE in preheated 375-degree oven for approximately 30 minutes. CHECK spanakopita while baking (stand away to avoid steam). As the top of dough begins to brown, DECREASE oven heat to 325. BAKE until golden brown (about one hour)
REMOVE from oven and let COOL for approximately 10 minutes. FLIP onto a baking sheet (jelly roll-style pan). PLACE back into oven to brown for another 10 minutes.
COOL (so the spinach/cheese mixture doesn’t leak out). CUT into squares and SERVE.