Heat 3 tablespoons oil in a large Dutch oven over medium heat. Add onion to pan; cook 5 minutes or until browned, stirring occasionally. Add spinach, 1 bag at a time; cook 3 minutes or until spinach wilts, stirring frequently. Simmer spinach mixture 40 minutes or until liquid evaporates. Stir in raisins. Remove from heat; cool completely. Stir in cheese, nuts, salt, and pepper.
Press 1 phyllo sheet into bottom and up sides of a 13 x 9–inch baking dish coated with cooking spray (cover remaining dough to keep from drying); lightly coat phyllo with cooking spray. Repeat procedure with 7 phyllo sheets. Spread spinach mixture in an even layer onto phyllo. Place 1 phyllo sheet on a large cutting board or work surface (cover remaining dough to keep from drying); lightly brush with 1 1/2 teaspoons oil. Repeat procedure with the remaining 3 phyllo sheets and the remaining 1 1/2 tablespoons oil. Place phyllo layer over spinach mixture; tuck in sides to enclose spinach fully. Bake at 400° for 30 minutes. Remove from oven; let stand 15 minutes.