It will last a year in a sealed container as powder! Spices generally don't go bad so much as they just lose potency. Also I would recommend using a mandolin or food processor to thin-slice the onions, I did them way too thick & they took a bit too long lol. Still came out find, but I couldn't use my APO for other stuff! So really thin = shorter time = frees up your APO, lol.
I took a spice class a few years back that really opened my eyes to why Columbus was so excited about sailing for spices lol. The chef said that he generally only keeps his fresh spices for 6 weeks & tries to use them up within that timeframe & then makes a new batch, because apparently the potency has sharp drop-off in flavor within a month & a half, as a rule of thumb.
So I just aim to make small batches of spices now. I have a mortar & pestle, a Krups grinder, and a Blendtec (big jar & small jar). You can do all kinds of homemade spices! One of my friends has a bay leaf plant, so she's giving me the harvest to split with her so we can both use them dried for cooking! I have some garlic going in tonight & some jalapenos the next day!
This batch was sweet onions, so the powder was sweet. But it's kind of like pumpkin, which is bland on it's own but really shines with some sugar & supporting spices like nutmeg & all-spice. So my base mix is Kosher salt & MSG (if you haven't gotten into MSG, read up on it! Not scary at all! I regret not using it all these years!) & then I'll mix the rest of it with the garlic powder & jalapeno powder to make a really nice pork rub.
Also Amazon sells various shaker bottles for spices with lids & holes, if you want to get into overnight dehydrating (which is literally as easy as falling off a log...slice your ingredient thin, stack uniformly across a rack in the oven...and do nothing while it dries lol). That way you can have fresh spices, spice mixes, custom rubs, etc. to enhance flavor, put aging food to good use instead of letting it go to waste, etc.
Super easy to toss in sliced apples, watermelon (try watermelon jerky!), leftover meats, herbs, veggies, etc. in there, because why not? I picked up this set of trays, which work great for dehydrating, which fit perfectly in the APO:
Here are some parchment sheets for catching drips, if you want easy cleanup: (I also use these for baking, flash-freezing, etc.)
Once I get my big list of stuff to try with the APO cleared out, this will probably be my schedule:
Looking forward to using & abusing this metallic monster! hahaha