Heat olive oil in a large skillet over medium-high heat. Add ground beef and chopped onion. Cook, breaking up the beef with a spoon, until the beef is browned and the onion is softened, about 5-7 minutes. Drain any excess fat from the skillet.

Stir in the minced garlic and taco seasoning mix. Cook for 1 minute until fragrant. Pour in the water and bring to a simmer, cooking for about 2-3 minutes until the liquid has slightly reduced and thickened.

Stir in the undrained diced tomatoes with green chilies, rinsed black beans, and drained corn. Cook for 3-5 minutes, stirring occasionally, until everything is heated through.

Add the cooked rice to the skillet, stirring gently to combine all ingredients. Ensure the rice is fully incorporated and warmed through.

Sprinkle the shredded Mexican cheese blend evenly over the top of the skillet mixture. Cover the skillet with a lid and cook for 2-3 minutes, or until the cheese is melted and bubbly.

Remove from heat and let stand for a minute. Serve hot directly from the skillet or spoon into individual bowls. Garnish with your favorite optional toppings such as sour cream, diced avocado, fresh cilantro, salsa, or crushed tortilla chips.