Wash spinach until no sand remains. Place it in a bowl with 1 T salt and cover with water, drain and chop it. In a skillet, melt 1 T butter and saute the onion; let cool. Combine onion with eggs and cheeses, and mix throughly with the spinach. Cut and roll fillo dough to fit four individual casseroles, 12 layers for each serving. Brushing each layer of doug with melted butter; lay 6 layers on the bottom of each casserole. Fill with 1/4 the amount of spinach mixture, and top with 6 more layers of buttered dough. Bake in a 350F oven for 15 minutes or until pastry is golden brown.