Wash, cut and scald the dock. Chop the onion and fry it in a few tablespoons of oil. Once softened, add the dock and simmer for 5 minutes.
Butter an oven dish and put two phyllo pastry sheets on top. Distribute crumbled feta cheese and part of the dock filling on top.
Place another 2 phyllo pastry sheets and repeat until out of products. You can pour melted butter between the sheets if you like. Finish off with two phyllo pastry sheets and cut it into squares.
Bake the phyllo pastry until nicely puffed, browned and ready. Cover the phyllo pastry with a cloth to temper. Optionally, you can substitute the dock with spinach.