Cheesy Spinach and Bacon Stuffed Potatoes

large russet potatoes

cream cheese, softened

whole milk

unsalted butter

pkg. frozen chopped spinach, thawed, drained and squeezed dry

thick-cut bacon slices, cooked and crumbled

finely chopped yellow onion

kosher salt

black pepper

sharp Cheddar cheese, shredded (about 2 cups), divided

sour cream

sliced scallions