Preheat the oven to 375 degrees F. Spray a 9×13-inch baking dish with nonstick cooking spray.
Add the ground beef and diced onion to a large skillet over medium-high heat. Cook for 5–7 minutes, breaking the beef apart as it cooks. Drain any excess grease if needed.
Add the bell pepper and garlic. Cook for another 2–3 minutes, until the pepper begins to soften.
Stir in the Rotel, black beans, salsa, taco seasoning, and cumin. Cook for about 3 minutes, allowing the mixture to heat through.
Stir in the cooked rice until everything is evenly combined.
Transfer the mixture to the prepared baking dish and spread it evenly. Sprinkle the reduced-fat Mexican cheese over the top.
Bake uncovered for 15–20 minutes, until the cheese is melted and the casserole is hot and bubbly.
Let the casserole rest for about 5 minutes before serving. Sprinkle with fresh cilantro and add your favorite toppings.