lök på laxen

Etymology
Literally, "onion on the salmon".

Phrase

 * 1) even worse; an extra disadvantage

Usage notes
Usually in either of the constructions "[att] lägga lök på laxen" ([to] make things even worse, literally "[to] put onion on the salmon") or "som (extra) lök på laxen" (as something which makes things even worse, literally "as (extra) onion on the salmon")