Cook brown rice according to package directions using chicken or vegetable broth.

Preheat oven to 350°F (175°C) and grease a casserole dish.

Heat olive oil in a large skillet over medium heat.

Saute diced onion in the skillet until tender and translucent.

Add thinly sliced zucchini, chopped cooked chicken (if using), and sliced mushrooms to the skillet.

Season the mixture with cumin, salt, and cayenne pepper.

Saute until chicken is heated through and zucchini is browned.

In a large mixing bowl, combine the cooked rice, skillet mixture, rinsed and drained black beans, drained diced green chili peppers, and shredded carrot.

Mix in half of the shredded taco or Mexican cheese.

Transfer the mixture to the prepared casserole dish.

Sprinkle the remaining cheese over the top.

Cover the casserole dish with foil.

Bake for 30 minutes.

Remove the foil and continue baking for 10 more minutes, or until lightly browned and bubbly around the edges.