Preheat the oven to 375˚F. 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 to 8 minutes. Remove to a paper towel-lined plate, leaving the grease in the skillet.
Add the onion to the skillet with the grease and cook until slightly softened, about 4 minutes. Add the garlic and cook 1 minute more. Add the ground beef and cook, breaking it into small pieces with a wooden spoon, until no pink pieces remain, 6 to 8 minutes. Drain off the excess fat, if you like. Remove the skillet from the heat. Stir in the soup, milk, salt, and ½ teaspoon black pepper; mix until smooth. Fold in the spinach to wilt slightly. Stir in the corn.
Sprinkle the cheddar cheese evenly over the top of the casserole. Place the tater tots on top in a single layer, arranging them in concentric circles (or just toss them on and push them very close together). Sprinkle the top with the parmesan cheese and the additional 1/4 teaspoon of black pepper.
Bake until he tater tots are golden, 30 to 35 minutes. Remove from the oven and crumble the cooked bacon over top. Let rest 5 minutes. Top with chopped chives or scallions and serve.