Handkäse (literally: hand cheese) is a German regional sour milk cheese (often Harzer) and is a culinary speciality of Frankfurt and Offenbach am Main. It gets its name from the former way of producing it: forming it with your own hands.
It is a small, translucent, yellow cheese with a pungent aroma that many people find unpleasant. It is sometimes square, but more often round in shape.
Often served as an appetizer or as a snack with Apfelwein (aka Ebbelwoi), it is traditionally topped with chopped onions, locally known as "Handkäse mit Musik" (literally: hand cheese with music) — so called because of the sound of the resulting flatulence brought on by the onions. It is usually eaten with caraway on it (as seen in the first picture). Since many people in Germany do not like this spice, in a lot of areas it is served on the side. Some Hessians say that it is a sign of the quality of the establishment when you get your caraway in a separate dispenser. As a sign of this, in many restaurants you will find, in addition to the salt and pepper, a little pot for caraway seeds. Hessians delight in introducing foreigners to this delicacy and explaining the name's provenance.
An alternative theory of explaining "Musik" is that the vinegar and oil flasks were formerly given separately to the guests, and that when they hit each other, they made that sound.
Usually you won't get a fork for your Handkäse. You cut a bit off and then stab it with the knife and eat it. Alternately, it can be served with bread and butter — you put a bit of cheese on a piece of bread, then place a bit of "Musik" on it with the knife and then bite into the bread.
While you traditionally drink Apfelwein with it, white wine is also accepted in some areas like Rheinhessen. Sometimes there are variants without "Musik" available.