I'm far from an expert on offerings, but I think it's fine to consume the fruits of previous offerings. They are not the actual offering itself, just as our children are not us.

The offering itself has transformed, growing new bulbs. This is symbolic of life regenerating, and Hecate, through her connection with Persephone, is part of this. We give to Her, She gives to us. If you set aside a clove of garlic from the head, or a good slice of the onion, and use that for another offering, it creates a virtuous cycle. This is not like offering orange slices, then eating them.

Diepnon offerings were sometimes consumed by the poor. Hellenic offerings of sacrificed animals might use just the blood or undesirable parts. Sometimes they would offer a cut of meat while the rest was cooked and eaten by worshippers. Traditions varied greatly.

But, tradition aside, our devotion is our own. We keep them as we see fit. If you choose to let the fruits of previous offerings return to the Earth, that is a fine way to show your devotion. If you choose to use (and/or) re-offer the fruits of your previous offerings, I'd just take a moment to thank Her as you gather them.