Preheat oven to 375 degrees. Roughly chop spinach leaves; set aside. Heat butter in a nonstick skillet over medium heat. Add onion and garlic; cook until tender, about 3 minutes. Add spinach; cook until wilted and bright green, about 2 minutes. Transfer mixture to a colander to cool and drain. When mixture is cool enough to handle, press lightly with paper towels to remove excess moisture; set aside to cool.
In a medium bowl, combine ricotta, eggs and egg whites, parsley, dill, Parmesan, cumin, salt, and pepper. Stir in cooled spinach mixture; set aside.
Dampen a large kitchen towel with water; squeeze dry. Unroll phyllo; place on work surface. Cover with plastic wrap and then the kitchen towel. Lightly coat a 9-inch Pyrex pie plate with cooking spray. Remove 1 sheet of phyllo, keeping remaining phyllo covered; place in prepared baking dish. Coat lightly with cooking spray. Repeat with 7 more sheets of phyllo, spraying between each layer. Trim edges with scissors, leaving a slight overhang.
Spoon filling into the dish, spreading it evenly. Bake until golden brown and filling is set, about 1 hour. Cool for at least 15 minutes before cutting into wedges.