Heat the oil in a large skillet over medium-high heat. Add the onions and sauté until soft, about 4 to 5 minutes.
2 tbsp olive oil, 1 cup onion
Working in batches, add handfuls of spinach to the pan and cook, stirring often, until completely wilted. Continue doing this until all the spinach has been cooked and is wilted.
2 lbs spinach
Turn off the heat and stir in the dill. Transfer to a colander. Once cool enough to handle, use your hands or a wooden spatula to squeeze out excess liquid.
¼ cup dill
In a large bowl, lightly beat together the eggs. Stir in the cooled spinach mixture, feta, Parmesan, and salt. Stir until fully mixed.
4 large eggs, 8 oz feta, 2 tbsp Parmesan cheese, ¾ tsp Kosher salt
Use softened butter or oil to grease a 9"x13" baking dish.
Add a layer of phyllo dough to the bottom of the dish, allowing one side to go up the side of the dish. Brush melted butter over the phyllo. Repeat this process 7 more times, allowing the sides to go up the side of the dish, alternating between the long sides of the dish and the ends, as well. Be sure to butter each layer.
8 tbsp unsalted butter, 1 lb phyllo dough
Transfer the spinach and cheese filling on top of the phyllo. Carefully use your fingers to fold the dough along the sides of the dish back over the filling. Use a sharp knife to help loosen any dough that is stuck to the side of the dish. This keeps the filling from spilling over.
Add 8 more layers of phyllo over the top, buttering each layer as you go. (If your dough is larger than 9"x13", you can carefully cut it to 9x13, or just go ahead and use the larger size, and fold the edges back over the dough).
Use a thin sharp knife to score the dough into 9 squares, cutting through the top layers of phyllo, but not cutting all the way through. You can score the squares into diagonals, making 18 triangles, if desired.
Place the dish in the refrigerator to chill for 30 minutes. After about 15 minutes of chilling, preheat the oven to 375°F.
Bake, uncovered, for 35 to 45 minutes, until the dough is puffed and golden on top.