Heat oven to 375 degrees F. Coat 13 x 9 x 2-inch baking pan with nonstick cooking spray. Melt butter in large skillet over high heat. Add onion and reduce heat to medium; cook 5 minutes. Add chicken and cook 8 minutes, until no longer pink. Add spinach, salt, pepper, nutmeg and red pepper flakes; cook 5 minutes. Transfer to large bowl. Stir in cheese, eggs and oregano.
2.
Place a half sheet of phyllo in bottom of pan. Coat with cooking spray. Repeat with 7 more half sheets, coating each sheet. Spread spinach mixture over top. Repeat with remaining 8 half sheets. Cut top layer in 8 equal pieces.
3.
Bake at 375 degrees for 45 minutes, until top is browned. Recut to serve.