Preheat oven to 375 degrees F & wash spinach and place in boiling salted water for about 3 minutes. Drain and rinse in cold water. Squeeze out all excess moisture and set it aside.
Heat oil in a saucepan and add onion, spinach and dill. Cook on high for 3 minutes until onions soften. Remove and cool. Combine the cooled spinach mixture with feta, eggs and salt and pepper. Set aside.
Work with ONE sheet of phyllo at a time. Cut each sheet of phyllo into 5 long, equal strips. Brush one strip with melted butter. Cover with a second strip and butter again. Lay strip down vertically in front of you. Spoon a heaping teaspoon of filling in bottom corner of strip. Fold over in half diagonally creating a triangle. Continue folding over into triangular shape until you reach end of strip. Repeat with remaining strips. Brush tops with a little more butter. Place triangles on a baking sheet and bake for about 15 minutes until golden.