01 - Preheat oven to 375°F. Lightly grease a 9x13-inch casserole dish and set aside.
02 - In a large skillet over medium heat, add a small amount of oil. Sauté the diced onion and bell pepper until softened, about 4 minutes. Add minced garlic and cook for 1 additional minute.
03 - Add black beans, corn, undrained diced tomatoes with green chilies, taco seasoning, salt, and black pepper to the skillet. Stir well and cook for 2 to 3 minutes until heated through.
04 - Stir in shredded chicken and sour cream. Mix thoroughly until all ingredients are evenly combined and the mixture is creamy and warmed.
05 - Remove skillet from heat. Stir in 1 cup shredded cheddar cheese and 1/2 cup Monterey Jack cheese until partially melted and evenly distributed.
06 - Spoon the prepared mixture into the greased casserole dish and smooth the top into an even layer.
07 - Sprinkle remaining cheddar cheese and Monterey Jack cheese evenly over the top.
08 - Place dish in oven and bake for 20 to 25 minutes, until casserole is bubbling and the cheese begins to turn golden.
09 - Remove from oven and allow to rest for 5 minutes. Garnish with sliced green onions, cilantro, diced avocado, or sliced jalapeños as desired. Serve warm.