Make the spinach filling: In a large skillet, combine olive oil, onion, garlic, and salt. Place over medium heat and cook for 3 to 4 minutes, or until the onion turns translucent and softens.
If using frozen spinach: Move your skillet off the heat. Drain the frozen spinach very well and squeeze out any excess water, then add the spinach to the skillet along with the lemon juice and feta cheese. Toss to combine, and set aside to prepare the crust.If using fresh spinach: Wash and drain the spinach well. Add the spinach leaves to the skillet, and continue to cook over medium heat until the spinach is wilted and all of the liquid evaporates (about 5 to 10 minutes). Cool the spinach mixture slightly (for about 10 minutes), then add the lemon juice and feta cheese. Toss to combine, and set aside to prepare the crust.
Preheat your oven to 325°F.
Start layering: Spread some of the melted butter (about 1 tablespoon) along the bottom of a 9x13 inch baking dish. Place 2 layers of phyllo dough on top. The standard size for phyllo dough is about 12 x 18 inches (which is how large my sheets were). So you may need to fold or cut each sheet in half to fit into the baking dish.
Spread another layer of butter over the phyllo. Top with 2 more layers of phyllo dough. Repeat the layering process, alternating between a layer of butter and 2 layers of phyllo dough, until you have about 16 layers of dough (or 8 large sheets folded). Do not top the last layer of phyllo with butter.
Evenly spread half of the spinach mixture over the last layer of phyllo dough. Top the spinach layer with 2 layers of phyllo dough, followed by a butter layer, and 2 more sheets of phyllo. Top with the remaining half of the spinach mixture.
Repeat the layering process again with 2 sheets of phyllo, followed by a brushing of melted butter, until all of the phyllo dough is used up. Top the assembled spanakopita with a final layer of butter.
Cut the spanakopita into your desired piece sizes (it's much, MUCH easier to cut before baking!) I like to go with 12 portions, but you can cut them smaller or larger if you like.
Bake in the center of your preheated 325°F oven for 45 to 50 minutes, or until golden brown on top. Cool slightly before serving!