Heat the oven to 375°F. Lightly grease 8 cups of a standard muffin tin.
Add the ground turkey, egg, oats, celery, onion, ¼ cup of the buffalo sauce, garlic powder, and salt to a mixing bowl. Mix gently until just combined.
Divide the turkey mixture among the 8 prepared muffin cups. Bake until the muffins are cooked through and reach 165°F in the center, about 20 to 25 minutes.
While the muffins bake, steam the cauliflower florets until very tender, about 12 to 15 minutes. Drain well, then transfer them to a bowl.
Add the butter and Greek yogurt to the cauliflower. Mash until smooth, or blend for a creamier texture.
Brush the baked muffins with the remaining buffalo sauce. Serve with the cauliflower mash and sprinkle the chopped green onions over the top.