Preheat oven to 350 degrees F. Spray a 9x13-inch casserole dish generously with cooking spray.
In a medium bowl, combine biscuit mix and cold water. Mix with hands until cohesive dough forms. Press dough evenly into prepared casserole dish. Bake for 5 to 8 minutes until set and lightly golden. Remove and set aside. Keep oven on.
Heat large skillet over medium-high heat. Add ground beef and cook 5 to 6 minutes, breaking apart with spoon, until browned. Drain excess fat.
Add chopped onion and red bell pepper to skillet with beef. Cook 4 minutes until vegetables soften. Add taco seasoning and stir well. Cook 1 minute more. Remove from heat.
Spread seasoned beef mixture evenly over par-baked biscuit base.
In a medium bowl, combine sour cream, mayonnaise, drained green chiles, 3/4 cup shredded cheese, and garlic powder. Mix until smooth. Spread carefully over beef layer.
Drop small spoonfuls of salsa randomly over sour cream layer. Sprinkle remaining 3/4 cup cheese evenly over top.
Bake uncovered for 30 minutes until cheese is melted, bubbly, and golden brown. Let cool 5 minutes before serving. Garnish with cilantro if desired.