Heat oven to 375 degrees F.
Divide the spinach into two extra large bowls. Season with salt in layers.
Heat an 8-quart pot over medium heat. Add the oil and onions. Season with salt and pepper. Saute until tender, 5 - 6 minutes.
Add the spinach into the pot, cover, and steam for 2 - 3 minutes. Toss and rotate until fully wilted, 1 - 2 minutes.
Place the spinach into a strainer lined with cheesecloth to cool. Squeeze out excess moisture.
In a large bowl, combine cooled spinach, feta, parsley, dill, and season to taste. Stir in the eggs until well combined.
Working with one filo sheet at a time, brush with butter, fold in half, and add ¼ cup of the spinach feta mixture to the top right corner.
Fold the filo over the mixture to create a triangle. Repeat with remaining sheets.
Place onto a rimmed baking sheet and brush the tops with butter.
Bake until golden brown and crisp, 22 - 25 minutes.
