Heat the oven to 375°F and lightly grease a 9-by-13-inch baking dish. If using raw chicken, cook it until it reaches an internal temperature of 165°F, then shred it; rotisserie or leftover cooked chicken can be used directly.

In a large bowl, stir the softened cream cheese, Buffalo sauce, ranch dressing, and 1/2 cup of the shredded cheddar until smooth and well combined. Mixing the creamy ingredients before adding the chicken helps prevent cream cheese lumps.

Fold the shredded chicken into the Buffalo cream cheese mixture until every piece is evenly coated. Spread the mixture in the prepared baking dish and smooth the surface.

Sprinkle the remaining 1/2 cup cheddar cheese evenly over the chicken layer. Arrange the frozen tater tots in a single layer over the top, leaving a little space between pieces when possible.

Bake the casserole uncovered for 40–50 minutes, until the filling is hot and bubbling and the tater tots are crisp and golden. For more browning, broil for 2–3 minutes at the end while watching closely.

Let the casserole rest briefly before garnishing with sliced green onions and crumbled blue cheese. Serve hot with extra ranch dressing or Buffalo sauce on the side.