Preheat oven to 375°F and grease a 9×13-inch baking dish.
Drain and rinse the canned black beans and set them aside.
Heat the 1 tablespoon olive oil in a large skillet over medium-high heat.
Add the minced garlic clove and cook 1 minute, stirring. Add the diced yellow onion and diced red and green bell peppers and cook about 5 minutes, until softened.
Add the drained black beans and 1 tablespoon taco seasoning to the skillet, toss to combine, and cook until warmed through. Remove from heat.
Place two 8" whole wheat tortillas in the bottom of the prepared baking dish, overlapping as needed to cover the bottom.
Spread half of the bean mixture evenly over the tortillas. Top with half of the cooked shredded chicken, pour half of the enchilada sauce evenly over the chicken, and sprinkle half of the shredded low-fat Mexican blend cheese over the top.
Place the remaining two tortillas on top. Repeat the layering with the remaining bean mixture, the remaining shredded chicken, the remaining enchilada sauce, and finish by sprinkling the remaining shredded cheese in a thick layer.
Cover the baking dish (with foil or a lid) and bake at 375°F for 15 minutes. Remove the cover and bake an additional 5 minutes, until the cheese is melted and lightly golden.
Let the casserole cool a few minutes to set, then top with the chopped green onion and serve.