The Red Onion is given as a 'reward' if Zagreus should fail a Perfect Clear challenge within Erebus, in place of whatever reward he would have received for succeeding. When consumed, it restores a measly 1 .
||'...One of the many ironies of living, if it can thus be called, in this eternal realm of gloom, is that the onion -- among every sustenance discovered -- ranks among the sole reminders here of our once-mortal diet; a foul root best known for bitterness or for unprompted issuance of tears. Yes, there are those who claim the onion has its place among the staples that a mortal ought to eat, deliberately, and with joy; for instance, they would say, the onion takes on different savor and consistency when put under the flame! But these are likely falsehoods to induce in us surpassing doubts, and make us weak. I would prefer to fade to nothingness far more than be required to consume one of those foul plants, which tainted too many a meal of mine while I was living, and goad me even now that I am dead.'