It isn't an actual walnut in a shell — it's a small cake baked in a walnut-shaped mold from batter mixed with ground walnuts, then filled with sweet red bean paste and topped with a single walnut half.
Batter made from wheat flour and ground walnuts is poured into walnut-shaped molds, filled with sweet red bean paste, and baked until golden, producing a small cake that looks exactly like an oversized walnut. Vendors near Cheonan Station and along the city's streets sell them warm in paper bags, a dozen or two at a time.
Eomuk / Odeng (어묵)
Korean Corn Dog (핫도그)
Ssiat Hotteok (Seed Hotteok)