Wash the spinach and chop it finely. Add 1 tablespoon salt and rub it in with the hands. Leave for an hour. Squeeze it well. Add 1/3 cup olive oil, the onions, the feta cheese crumbled, eggs, parsley, dill and pepper. Take a buttered baking pan 10'' X 14'' X 2'' and line with one pastry sheet. Brush liberally with oil. Add 6 more pastry sheets, brushing each with oil. Fill with spinach mixture and cover with more sheets of filo pastry. Brush the top sheet with oil and carve the top three sheets with a sharp knife into 3-inch strips from one end of the pan to the other. Sprinkle the top with water, to prevent the pastry sheets from curling upwards. Bake the spinach pie in a moderate oven for 40-45 minutes. Cool slightly and cut into squares.
Serve hot or cold. Makes 20 pieces.