A metaphor Dan Olsen uses for uncovering the specific customer benefits nested inside a generic, high-level market problem statement, by peeling back layers the way you'd peel an onion. A statement like "help me prepare my taxes" is an outer layer; asking what's underneath it repeatedly exposes more detailed, atomic benefits (avoid math errors, feel confident about audit risk, minimize time spent) until you reach something specific enough to act on.
The Onion Model works in the opposite direction from Benefit Ladder's "why" climb: Onion peels downward, from a vague high-level problem into its specific constituent benefits, while the benefit ladder climbs upward, from a specific stated benefit toward the macro purpose it serves. Used together they bound a benefit's place in Problem Space vs. Solution Space work — down to find what's concretely underneath a generic problem, up to find what it's ultimately in service of.