Soak rice cake slices in cold water for 1 hour while preparing beef.

With a sharp knife, slice beef into thin strips.

Combine soy sauce, sesame seeds, garlic, salt, and black pepper in a small bowl.

Mix beef and soy sauce mixture together.

In a large soup kettle, stir-fry beef until meat begins to brown.

Add broth to beef mixture in the kettle.

Bring to a boil, then lower heat.

Simmer, uncovered, for 45 minutes.

To keep your hands from sticking to the rice cakes, wash thoroughly, being careful to get under your fingernails.

Using your clean hands, separate each individual rice cake from the frozen clumps; this helps rice cakes cook evenly.

Add rice cakes to the soup along with sliced onions.

Cook just until rice cakes rise to the surface of the soup, 20 to 25 minutes.

Do not overcook; rice cakes should be chewy.

Pour beaten egg into the bubbling soup; stir well.

The egg will cook immediately.

Serve hot.