Mexican Fiesta Casserole

Name: Mexican Fiesta Casserole
Ingredients: 2 teaspoons canola oil; 1/2 pound lean ground turkey; 1 cup finely chopped onions; 1 cup finely chopped zucchini; 1 envelope (1 1/4 ounces) low-sodium taco seasoning; 2/3 cup frozen corn kernels; 1 cup canned black beans, rinsed and drained; 2 cups jarred salsa; 8 (6-inch) corn tortillas; 1 1/2 cups shredded reduced-fat Mexican cheese blend; 1/4 cup sliced scallions (white and light green parts only); 1/2 cup fat-free sour cream (or use low-fat; optional), for serving