Preheat oven to 350°F. Spray bottom and sides of round baking dish such as a quiche plate with nonstick cooking spray. Arrange four tortillas in bottom of baker, overlapping slightly.
In large (12-in.) skillet, cook ground beef over medium heat 8-10 minutes or until no longer pink, breaking meat into small crumbles. Drain, if necessary. Add taco seasoning mix and water; cook according to package directions. Remove from heat; stir in 1 cup of the salsa.
Spoon beef mixture evenly over tortillas in baker. Sprinkle with 1 cup of the cheese. Sprinkle half of the green onion over cheese. Top with remaining four tortillas, overlapping slightly and pressing down lightly. Spread remaining salsa over tortillas. Sprinkle with remaining cheese and onion.
Bake 28-30 minutes or until cheese is melted. Remove from oven; let stand 5 minutes. Cut into wedges. Serve with lettuce and sour cream.