Pre-heat an oven to 350 Degrees F.

Brush a light layer of butter on the base and sides of a 9X13 casserole dish or pan.

Unwrap fillo dough and lay it on a clean work table; Place a kitchen towel over the stack of fillo to prevent the sheets from drying while assembling the pie.

Place one sheet of fillo inside the pan and evenly brush the sheet with a layer of melted butter; In the opposite direction, place another sheet of fillo in the pan and brush with a layer of melted butter.

Repeat step 4 until you have a total of 6 layers.

Evenly spread the spanakopita filling across the pan.

Repeat step 4 until you have another 6 layers.

Fold the corners of the fillo back into the pan and brush a final layer of butter across the top of the pie.

Using a sharp knife, score 12 slices (only cutting through the top layer of fillo)

Bake the spanakopita for 50 minutes on the middle rack or until pie is completely golden.

Allow the pie to cool for 10 minutes before cutting and serving.

Set a medium saute pan over a burner set to medium-high heat.

Add vegetable oil, garlic and onions to the pan and saute until until onions are transulcent and garlic is fragrant.

Allow the mixture to cool for 10 minutes before proceeding.

Add spinach to a fine mesh strainer or wrap in cheese cloth and set over a medium mixing bowl, squeeze the spinach as much as possible to get rid of excess water.

Break up spinach in a large mixing bowl and add remaining ingredients; Fold until well incorporated.

Transfer to an airtight container and refrigerate until needed.