In a large saucepan, bring water to a boil; add asparagus. Cover and cook for 4-6 minutes or until crisp-tender. Drain and immediately rinse with cold water; drain and pat dry. In a large bowl, combine the asparagus, cheeses, onion, garlic powder, salt and pepper.
Brush a 15-in. x 10-in. x 1-in. baking pan with some of the butter. Unroll phyllo sheets; trim short side to fit pan. Cover dough with plastic wrap and a damp cloth while assembling. Place two sheets of phyllo in pan; brush with butter. Repeat eight times. Spread with asparagus mixture. Layer with remaining phyllo sheets, brushing each with butter.
Bake at 350° for 40-45 minutes or until golden brown. Cool on a wire rack for 5 minutes. Cut into 24 pieces; cut each piece in half diagonally.