In a small saucepan (or in the microwave), melt the butter over low heat, then add the milk, sugar and salt. Stir to combine, then shut off the heat. Let the mixture cool until it's just warm, then transfer to a large mixing bowl.
Add the egg, yeast, flour and potato flour (if using) and mix to form a dough. Turn it out on to a clean work surface and knead until smooth - about 5 minutes. You can also do this in a mixer with a dough hook if you wish. The dough should be a bit tacky, but if it's too sticky to knead, add more flour 1 tablespoon at a time until it's no longer sticking.
Rub the mixing bowl with a bit of neutral oil, then place the dough ball inside and cover. Place in a warm spot and let rise for about 90 minutes, or until puffed up (but not doubled in size).
While the dough is rising, mix together the cream cheese and scallions until combined. Set aside (do not refrigerate to keep it spreadable).
When the dough has risen, punch it down, then place it on a clean work surface and use a rolling pin to roll it out to roughly 20 x 12 inches.
Spread the cream cheese mixture all over the dough right up to the edges. I find an offset spatula does this the easiest.
Cut the dough into 5 strips that are each 12 inches long and 4 inches wide. They don't have to be exact, but try to get them as evenly sized as possible.
Stack the strips on top of each other (cream cheese side up), then cut the stack into 6 segments roughly 2x4 inches. Again, try to get them to all be the same size as best as you can.
Cover the loaf loosely, then let rise for another 30 minutes, or until puffy.
Preheat the oven to 350 degrees, then butter the inside of a 9-inch loaf pan. Carefully lay each stack of dough on it's side and place them in the pan next to one another. You may have to move them around to squeeze them all in - that's okay, it will bake up fine.
Brush the top with melted butter, then sprinkle all over with everything bagel seasoning. Use as much or as little as you like. I used a ¼ cup (4 tablespoons) because I love a lot of seasoning.
Bake for about 30 minutes, or until it's golden brown all over.
Let the bread cool for about 10 minutes, then carefully remove the bread from the pan. Serve warm.