Preheat and Prepare: Preheat your oven to 400°F (200°C). Lightly grease a 9x13 inch baking dish.
Mix the Buffalo Chicken Filling: In a large bowl, combine the shredded chicken, softened cream cheese, buffalo sauce, and ranch or blue cheese dressing. Mix until everything is well combined and creamy. Stir in 1/2 cup of the cheddar cheese, 1/2 cup of the mozzarella cheese, and the chopped green onions.
Assemble the Casserole: Spread the buffalo chicken mixture evenly into the bottom of the prepared baking dish.
Add the Tater Tots: Arrange the frozen tater tots in a single, even layer over the chicken mixture. For best results, do not thaw the tater tots first; placing them on top while frozen helps them get extra crispy during baking and prevents the casserole from becoming soggy.
Bake and Add Cheese: Bake uncovered for 25-30 minutes, or until the tater tots are starting to turn golden brown and the filling is bubbly. Carefully remove the casserole from the oven and sprinkle the remaining 1/2 cup of cheddar and 1/2 cup of mozzarella cheese over the top.
Final Bake and Rest: Return the casserole to the oven and bake for another 5-10 minutes, or until the cheese is completely melted and bubbly and the tots are golden brown. Let it rest for 5-10 minutes before serving. Garnish with additional green onions and a drizzle of ranch or blue cheese dressing if desired.