Preheat the oven to 375 degrees. Coat a 9 x 13 inch baking dish with cooking spray and set aside.

Prepare the enchilada sauce. Heat the oil in a medium pot over high heat. When the oil is shimmering, add the arrowroot powder. Reduce the heat to medium and whisk until combined, about 1 minute. Add the tomato sauce, broth, cumin, chili powder, garlic powder, and salt. Increase the heat to high to bring to a simmer, then reduce the heat to low and cook, stirring occasionally, for 15 to 20 minutes, until thickened.

While enchilada sauce cooks, spiralize the zucchinis. Slice halfway through the zucchinis lengthwise, careful not to pierce through the center. Spiralize on Blade A to yield chip slices. Set aside.

Spread ½ cup of the enchilada sauce on the bottom of the baking dish. Arrange zucchini slices in a single layer, overlapping slightly to cover up any little gaps. Sprinkle over with half of the beans, half of the chicken, 1 cup of the cheese, ½ cup of the enchilada sauce, and half of the spinach. Repeat starting with laying out the zucchini slices and then the beans, chicken, cheese, enchilada sauce, and spinach. Finish the casserole with the remaining zucchini and enchilada sauce.

Bake the casserole for 20 minutes, and sprinkle with remaining 1 cup of cheese. Bake until the cheese melts and casserole bubbles, about 5 minutes. Let cool for at least 10 minutes before serving. You should expect excess moisture (because zucchini is very water dense), so you can pour out the liquid before serving, if you wish. Otherwise, embrace it!

Drizzle the top of the casserole with the remaining sauce. Top with avocado and cilantro. Serve.