Preheat the oven to 375 degrees. Remove the tater tots from the freezer to let them thaw slightly while you prepare the filling. Heat a large cast iron skillet over medium heat. Add the bacon and cook until golden brown and extra crispy, 6-8 minutes. Remove to a paper towel lined plate and leave the grease in the skillet. Set all of the bacon aside for topping later.
Add the onion to the skillet and cook until slightly softened. Add the garlic and cook 1 more minute. Add the ground beef and break into small pieces. Cook the meat until no longer pink. Drain off excess fat or liquid. Remove skillet from the heat. Stir in the soup, milk, salt and ½ teaspoon black pepper and mix until smooth. Fold in the spinach to wilt slightly, then the corn.
Sprinkle the cheddar cheese evenly over the top of the casserole. Place the tater tots on top in a single layer. Sprinkle the top with Parmesan cheese and the additional 1/4 teaspoon of black pepper.
Bake 30-35 minutes until the tater tots are golden. Remove from the oven and crumble the cooked bacon over top. Let rest 5 minutes. Top with chopped chives