Heat vegetable oil in a large saucepan over medium heat. Slowly cook and stir onions until softened. Mix in spinach, dill and flour. Cook approximately 10 minutes or until most of the moisture has been absorbed. Remove from heat. Mix in feta cheese, eggs, salt and pepper.
Separate one sheet of filo from the stack and evenly brush with a light coating of butter. Place another sheet of filo over the butter and press the two sheets together. Cut the layered filo dough into long strips about 8cm wide. Keep the remaining filo covered with plastic wrap to keep it from drying out.
Lay out one strip of filo at a time on your work surface with one of the narrow ends close to you. Place a heaping tablespoon of filling 2cm from the end closest to you. Fold the bottom right corner over the filling to the left edge to form a triangle. Fold the triangle up, bringing the point at the bottom left up to rest along the left edge. Turn the lower left corner over to touch the right edge. Continue turning the triangle over in this manner until you reach the end of the filo. Repeat with the remaining filling and filo dough.
Place the filled filo triangles on a large baking sheet and brush with the remaining butter. Bake in the preheated oven until golden brown, 45 minutes to 1 hour.