This is my current favorite grilled cheese, with a high satisfaction to effort ratio. Some extra-special grilled cheeses make kind of a pain in the ass out of what's supposed to be an easy meal (and hey, I cook those sometimes too). But this one has all the panache and minimal fuss.
Spread some mustard on the inside of each slice of bread. Arrange one slice of American and half the cheddar in a single layer on each slice of bread, making sure to distribute the cheddar like a cheese boss who gets what she wants out of every bite. Lay the sauerkraut on the bottom slice and top with the top slice. Spread half the butter over the outside of the top slice of bread.
Place the sandwich buttered side down in a skillet over medium-high heat. Butter the outside of the top slice with the remaining butter. Cook, adjusting heat as necessary, until bread is golden brown and cheese is melted, flipping once halfway through. Cut and serve immediately.
My favorite sauerkraut these days is Deep Root Organic, which is all raw and probiotic and studded with magic caraway seeds.