Preheat the oven to 350°F then prep the casserole ingredients. Strain and rinse the black beans. Using a vegetable chopper, dice the peppers and onions.
Then, in a mixing bowl, add cooked and shredded chicken, sour cream, the juice of one lime, and taco seasoning. Stir together with a rubber spatula.
Spray a casserole dish with cooking spray. Pour in black beans, peppers, and onions. Mix to combine.
Pour in the chicken and sour cream mixture and spread it evenly over the black bean and vegetable base.
Sprinkle the remaining cheddar cheese on top. Garnish with green onion.
Bake for 28-30 minutes or until the cheese is bubbly and golden brown. Serve with dollops of sour cream, and avocado slices.