Oven Baked Onion Rings

Cooking spray 4 cups Ritz crackers 1/2 teaspoon cayenne 1 cup low-fat buttermilk 1/2 cup plus 2 tablespoons all-purpose flour 1/2 teaspoon salt 1/4 teaspoon fresh ground black pepper 1 to 2 large Vidalia onions, peeled