Preheat oven to 400°F. Spray a large (13x18-inch) baking sheet with cooking spray. Slice the green onions.
Cover the bottom of the baking sheet with half of the flour tortillas, trimming to fit. Sprinkle with the cheese, the diced bell peppers, green chilies, and half the green onions. Top with remaining tortillas, trimming to fit. Bake for 10 min. or until golden brown and cheese is melted. Cut into squares and garnish with remaining green onions.