Place salmon fillet on a broiler pan coated with cooking spray. Sprinkle salt, garlic powder, and pepper evenly over fillet. Broil 10 minutes or until fish flakes easily when tested with a fork. Cut fillet into small chunks; set aside.
Preheat oven to 350°.
Combine potato, milk, and leek in a small saucepan; bring to a boil. Cover, reduce heat, and simmer 10 minutes or until potato is tender. Combine water and cornstarch in a small bowl. Stir well; add to pan. Bring to a boil; cook 1 minute, stirring constantly. Gently stir in fish chunks, dill, and cheese. Set aside.
Place 1 phyllo sheet on work surface (cover remaining dough to keep from drying); lightly coat with cooking spray. Working with 1 phyllo sheet at a time, coat remaining 7 phyllo sheets with cooking spray, placing one on top of the other. Place a sheet of plastic wrap over phyllo, pressing gently to seal sheets together; discard plastic wrap.
Spoon potato mixture along 1 long edge of phyllo, leaving a 2-inch border. Fold over the short edges of phyllo to cover 2 inches of potato mixture on each end.
Starting at long edge with 2-inch border, roll up jelly-roll fashion. (Do not roll tightly, or strudel may split.) Place strudel, seam side down, on a jelly-roll pan coated with cooking spray. Score diagonal slits into top of strudel using a sharp knife. Lightly spray strudel with cooking spray.
Bake strudel at 350° for 30 minutes or until golden brown. Serve warm.