To make filling: Place spinach with water still clinging to the leaves in a large pot. Cover and cook over medium heat until the spinach is wilted, about 5 minutes. Drain and refresh with cold water. Squeeze the spinach dry and chop.Advertisement
Heat oil in a nonstick skillet over medium heat. Add scallions and saute until softened, 2 to 3 minutes. Transfer to a medium bowl and stir in spinach, feta, Parmesan, dill and lemon juice. Season with salt and pepper. Stir egg whites into the spinach mixture.
To form phyllo rolls: Set oven rack on the upper level; preheat to 350 degrees F. Lightly coat a baking sheet with cooking spray or line with parchment paper.
Whisk together egg white, oil and salt in a small bowl.
Lay one sheet of phyllo on a work surface with a short side toward you. Brush lower half of the sheet with the egg-white mixture and fold top half over. Brush the top with the egg-white mixture. Repeat this step with a second sheet of phyllo and set on top of the first. Spoon one-quarter of the spinach filling along one long edge. Tuck in the side edges and roll up, jelly-roll fashion. Place on the prepared baking sheet. Repeat with the remaining phyllo, egg-white mixture and filling, making 4 rolls in all.
Brush tops of the rolls lightly with the egg-white mixture and sprinkle with seeds, if desired. Bake until golden, 25 to 30 minutes. Cool for 5 minutes. With a serrated knife, cut each roll diagonally into 9 pieces and serve hot.
Make Ahead Tip: Cover and refrigerate up to 2 days. Reheat in a 350°F oven until heated through, 10 to 12 minutes.