5

That's because eating onions is forbidden not just during fasts, but at all times. Here's what this chapter of the Manu Smriti says:
laśunaṃ gṛñjanaṃ caiva palāṇḍuṃ kavakāni ca |
abhakṣyāṇi dvijātīnāmamedhyaprabhavāni ca ||
Garlic, leeks and onions, mushrooms and (all plants), spring...