Preheat the oven to 350°F (175°C).
Cook the elbow macaroni according to package instructions until al dente. Drain and set aside.
In a large skillet, heat olive oil over medium heat. Add the diced onion and cook for 3-4 minutes until softened.
Add the ground beef to the skillet with the onions. Cook, breaking it up with a spoon, until browned and cooked through, about 6-8 minutes.
Add the minced garlic and cook for 1 minute until fragrant.
Stir in the taco seasoning mix and 1/2 cup water. Simmer for 3-4 minutes until the sauce thickens slightly.
Remove the skillet from heat and stir in the salsa, sour cream, milk, black beans, and corn. Mix well to combine.
In a large mixing bowl, combine the cooked macaroni and the beef mixture. Stir in 1/2 cup of shredded cheddar cheese and 1/2 cup of shredded Monterey Jack cheese. Season with salt and black pepper to taste.
Transfer the mixture to a 9×13-inch baking dish. Sprinkle the remaining cheddar and Monterey Jack cheese evenly over the top.
Cover the dish with aluminum foil and refrigerate if making ahead, or bake immediately.
To bake immediately: Bake covered for 20 minutes. Remove the foil and bake for an additional 5 minutes until the cheese is melted and bubbly.
If baking from refrigerated: Remove foil and bake at 350°F (175°C) for 25-30 minutes until heated through and cheese is melted.
Remove from oven and let stand for 5 minutes before serving.
Garnish with chopped fresh cilantro if desired.