Cook ground beef in 2 batches. For first batch, spray 12-inch skillet with cooking spray. Add half of the ground beef and 1 chopped onion. Cook over medium heat 8 to 10 minutes or until beef is thoroughly cooked, stirring frequently. Drain.
Stir in 1 package taco seasoning mix, 3/4 cup salsa and 1/2 cup water. Reduce heat to low; simmer 8 to 10 minutes or until most of liquid is absorbed.
Cool first batch of beef mixture 15 minutes. Spoon into refrigerator or freezer container; cover tightly. Repeat to make second batch. Serve immediately, or refrigerate up to 24 hours or freeze up to 1 week.
To serve immediately, place beef mixture in slow cooker; keep warm on Low setting. If beef mixture is frozen, thaw before heating. Place beef mixture in slow cooker; cover and cook on High setting for 1 hour or until hot. Reduce heat to Low to keep warm. To serve, spoon beef mixture into taco shells. Top with Layered Taco Toppings.
To make Layered Taco Toppings, heat oven to 425°F. Spray 15x10x1-inch baking pan with nonstick cooking spray. Dry corn on paper towels; spread corn in sprayed pan. Bake at 425°F for 15 minutes. Stir; bake an additional 10 minutes. Cool 5 minutes or until slightly cooled.
In ungreased 13x9-inch (3-quart) glass baking dish, layer lettuce, bell pepper, roasted corn, onions, tomatoes and avocado.
In small bowl, combine mayonnaise and salsa; mix well. Spread over vegetables. Sprinkle with cheese and cilantro. Cover tightly; refrigerate at least 2 hours or overnight.