You return to your hand all cards making up the mutated creature.
The Mutation mechanic causes creature cards to merge into one new creature. They make up that new object - which of those cards is the top-most only matters for some of its characteristics.
When a mutated creature dies, all creatures that have been merged also go to the graveyard. If the mutated creature triggers Koll, and Koll's ability can find one of those creature cards, the ability affects all of those cards and they are all returned to your hand.
724.1. One keyword causes an object to merge with a permanent. See rule 702.140, “Mutate.”
724.3. If a merged permanent leaves the battlefield, one permanent leaves the battlefield and each of the individual components are put into the appropriate zone.
724.3c If an effect can find the new object that a merged permanent becomes as it leaves the battlefield, it finds all of those objects. (See rule 400.7.) If that effect causes actions to be taken upon those objects, the same actions are taken upon each of them.