Preheat oven to 375°F (190°C). Grease a 9x13-inch baking dish.
In a large skillet over medium-high heat, brown the ground beef along with the diced yellow onion. Cook until the beef is no longer pink (about 5-7 minutes). Drain excess grease.
Add the minced garlic, salt, and black pepper to the skillet. Cook for 1 minute until fragrant.
Stir in the cream of mushroom soup, sour cream, whole milk, drained corn kernels, and 1 cup of the shredded cheddar cheese. Mix until well combined.
Transfer the beef mixture to the prepared baking dish, spreading it out into an even layer.
Arrange the frozen tater tots in an even, single layer on top of the beef mixture.
Bake uncovered for 35 minutes, or until the tots are golden brown and crispy.
Remove from the oven and sprinkle the remaining 1 cup of cheddar cheese evenly over the top.
Return to the oven for 5 minutes, or until the cheese is completely melted and bubbly.
Remove from oven. Drizzle with ketchup and garnish with fresh parsley before serving warm.