Heat the oil in a non-stick pan over medium heat. Sauté the onion and garlic for 3 to 4 minutes. Add the spinach and cook until it is wilted. Season with salt and pepper. Remove from the heat and drain as much moisture out of the spinach as you can. Toss in the feta cheese.
Place the phyllo sheets on a parchment paper lined baking sheet so they are overlapping, based on the amount of filling you have. Lay the spinach mixture down the center of the phyllo sheets, fold in the ends and roll up like a burrito. Brush the outside with melted butter.
Bake in the oven until golden brown and crispy (about 20 minutes).
Slice and serve.
Video
Notes
Pro tip: Make multiple (2 or more) spanikopita instead of one big one if you have to make over 2 portions. According to the size of the phyllo sheets you may have to overlap the sheets to fit the amount of filling you have.