1Preheat oven to 375°F (190°C). Grease a 9×9-inch baking dish with olive oil or cooking spray.
2In a large skillet, heat olive oil over medium heat. Add the diced onion and minced garlic, and sauté for 2-3 minutes, or until fragrant. Add the ground beef and cook, breaking it apart, until browned and cooked through. Drain excess fat if needed.
3Stir in the taco seasoning and 1/4 cup water. Let the mixture simmer for 2-3 minutes until fully combined.
4Spread the cooked beef mixture evenly in the bottom of the prepared baking dish.
5Spoon sour cream over the beef layer and spread it out gently. Top with diced tomatoes, bell peppers, black olives (if using), and green onions.
6Sprinkle the shredded cheddar cheese evenly over the top of the casserole.
7Bake for 15-20 minutes or until the cheese is melted and bubbly.
8Remove from the oven, sprinkle with fresh cilantro if desired, and serve hot.