Preheat the oven to 350 degrees Fahrenheit. Butter a 13x9 baking dish, and set aside.
Heat the oil in a skillet over medium-high heat. Add the onions, and sauté until translucent. Add the spinach, garlic, a pinch of salt, and a pinch of black pepper to taste. Sauté for 1 minute.
1 Tablespoon olive oil, 1 medium onion, 2 (10 ounce) packages frozen spinach, 4 garlic cloves, Salt to taste, 1/2 teaspoon black pepper or to taste
Remove the skillet from the heat, and set aside to cool.
Once cool, add the feta and 1 cup Mozzarella cheese. Stir to combine.
1/2 cup feta cheese crumbles, 1 cup Mozzarella cheese
Layer two pieces of phyllo dough on top of each other, dot the spinach mixture on top, spreading it out evenly. Begin to crinkle the sheets, folding them over each other accordion-style.
1 package phyllo sheets
Lay the folded dough in the buttered baking dish, starting at the short end.
Repeat until all the phyllo has been used.
In a bowl, combine the eggs, milk, yogurt, baking powder, salt, pepper, and melted butter. Stir until smooth.
2 large eggs, 3/4 cup milk, 1/2 cup plain yogurt, 1 teaspoon baking powder, 1/4 cup unsalted butter
Pour the mixture on top of the phyllo dough. Sprinkle the remaining cheese on top.
Bake 30-35 minutes or until the cheese is melted and golden brown.
Store covered in the fridge for up to 3 days.