Thaw the phyllo and spinach: Remove both from the freezer at least 30 minutes before beginning.
Sauté the onions: Heat ¼ cup olive oil in a skillet over medium heat. Add red onion and scallions with a sprinkle of salt. Sauté for 1–2 minutes, until fragrant and softened. Set aside.
Preheat the oven: to 350°F.
Prepare the spinach: Place spinach in a colander. If any pieces are still frozen, run warm water over them. Drain well, then squeeze out as much excess water as possible—this step is key to prevent sogginess.
Make the filling: In a large bowl, beat the eggs. Add drained spinach, sautéed onions, dill, feta, and ¼ teaspoon salt. Mix well to combine.
Prepare the phyllo layers: Melt the butter in a small pan and stir in ½ cup olive oil. Brush the bottom of a 9x13-inch baking dish with the olive oil-butter mixture.
Layer the phyllo: Unroll the phyllo dough and carefully place one sheet in the dish. Brush with the olive oil-butter mixture. Repeat with about 12 sheets, brushing each with oil and butter.
Add the filling: Spread the spinach mixture evenly over the phyllo layers.
Top with remaining phyllo: Layer another 12 sheets on top, brushing each with the oil-butter mixture. Fold the edges over the sides and brush with more oil-butter to seal.
Bake: Place in the oven and bake for 40 minutes, or until the phyllo is golden brown and crispy.
Cool and serve: Let cool for about 15 minutes before slicing and serving.