Thaw spinach; drain well, pressing between paper towels.
Prepare grits according to package directions. Stir feta, next 5 ingredients, and spinach into grits. Add freshly ground pepper to taste.
Stir together panko and next 2 ingredients; sprinkle half of mixture into a well greased 13- x 9-inch baking dish. Pour hot grits mixture into prepared pan. Sprinkle remaining panko mixture over grits. Cover and chill 8 hours.
Invert chilled grits onto a cutting board; cut into 24 squares. Cut each diagonally into 2 triangles. Coat both sides well with cooking spray.
Cook triangles, in batches, on a hot greased griddle or in a nonstick skillet over medium heat 2 minutes on each side or until golden. Place on a wire rack on a baking sheet, and keep warm in a 200° oven up to 30 minutes.