Preheat oven to 350F
Place spinach in a colander and press firmly to get rid of most of the excess liquid
Saute onions and garlic in olive oil in a large pan. When the onions have softened stir in the spinach and season liberally with salt. Continue to saute for 8-10 minutes until most of the moisture is gone.
Put the spinach into a large mixing bowl and set aside to cool for a bit then stir in eggs, feta and parmesan.
Lay a phyllo sheet on a work surface, brush it liberally with olive oil, place another sheet on top and repeat so there are 4 layers.
Add 4 1 tbsp piles of spinach at one end of the dough, spread equally apart.
Use a sharp knife or pizza cutter to cut 4 strips
Fold each strip into a triangle shape, here's a video on the technique https://youtu.be/FVnbFR4SnGU?t=2m9s then place triangles in a greased baking pan.
Repeat 4-7 until you run out of sheets and filling.
Bake triangles at 350F for 30 minutes.