In a large skillet, heat the olive oil over medium heat. Add the minced garlic and diced onion, sautéing until the onion becomes translucent, about 3-4 minutes. Toss in the diced red and green bell peppers, cooking for an additional 2-3 minutes until softened.
Stir in the drained and rinsed black beans along with the taco seasoning. Mix well and cook for another 2 minutes, allowing the flavors to meld.
Preheat your oven to 350°F (175°C). In your 9x13 inch baking dish, layer 2 whole wheat tortillas, tearing as needed to cover the bottom.
On top of the tortillas, spread half of the chicken, followed by half of the vegetable and black bean mixture. Sprinkle 1 cup of cheese over this layer. Pour half of the enchilada sauce over the cheese.
Add another layer of 2 tortillas, the remaining chicken, the rest of the vegetable mixture, and another cup of cheese. Finish with the remaining enchilada sauce and top with the last cup of cheese.
Cover the baking dish with aluminum foil and bake for 20 minutes. After 20 minutes, remove the foil and bake for an additional 10-15 minutes, or until the cheese is bubbly and golden.
Once out of the oven, let it cool for a few minutes. Sprinkle with chopped green onions before serving.