Boil frozen spinach. Drain spinach very, very well. Make sure almost ALL water is removed. In a large bowl mix drained spinach, crumbled feta, 8 eggs, finely chopped yellow onion, chopped dill, salt & pepper to taste, and 1 tblsp olive oil.
Melt half stick butter in small skillet. Use melted butter to butter a 9" x 13" pan. Put down four layers of phillo dough (baste phillo with butter every other layer). Put above mixture in pan evenly distributed. Top with six layers of phillo dough (baste every other layer and make sure top layer is well basted). Place in pre-heated, 400 degree oven. Cook for 45-50 minutes.