In general, the issue is how long the food spends at the middle temperatures, warmer than refrigeration, and not as hot as truely safe cooking temperatures, the so called "danger zone" from 40-140 F, 4-60 C.
While an over simplification, pathogens do not grow well outside of the danger zone because it is so cold that they are inactive, or so hot that they are distressed or killed.
Slow cooking, done properly, is safe because the time spent moving through the danger zone to the safety of temperatures above about 140 F / 60 C is minimized, thus minimizing the opportunity for any pathogens to grow and create toxins; then, at temperatures above the threshold (if held for several minutes), most pathogens are killed. (Do not take this to mean food is "sterilized" as spores can remain, that can become active later.)
Foods are not cooked to a single temperature to be considered safe. Instead, there is a combination of time and temperature at which they are considered safe, assuming no other problems existed. This is mere seconds at 165 F / 74 C, but perhaps half an hour (going by memory, so don't take this as gospel) at 135 F / 57 C.
Still, general audience food safety recommendations suggest temperatures of 160 F / 71 C because the 99.9% kill time at that temperature is only a few seconds--it doesn't require careful monitoring or timing, and so is a simple rule to follow.
275 F / 135 C (I see you edited the question) is definitely a temperature range for slow roasting that is safe when done properly.
135 F / 57 C is possible, but on the borderline, although it would require very precise control not usually available in an oven; this is more the realm of sous-vide cooking techniques. It also requires longer periods held at temperature, and so is not a method for casual cooks.
100 F is not safe. It is well into the temperatures at which food is being incubated so the pathogens have a cozy environment for growth.
Onions shouldn't degrade that quickly at room temperature anyhow, even if they weren't partially dried. This food safety and preservation site specifically addresses inadequately dried foods:
Check containers within seven to 10 days to see if moisture is
present. If you see moisture, remove food and redry at 140 to 160
degrees Fahrenheit. If food is moldy, discard it, throw away the
plastic freezer bag or sterilize the jar.
So, if your onions were partially dried, it should be safe to continue drying them. If they weren't very dry yet, I'd treat them as fresh onions at room temperature, which should also be stable enough not to go bad overnight.
(By the way, I assume part of the motivation for this question may have to do with a well-known myth which was circulating a few years back about cut onions "attracting" bacteria. This is certainly false, and here's a food scientist's explanation in more detail. Unless you sliced the onions with a highly contaminated knife, there should be no danger.)
Best Answer
If you're really worried about it, put some superglue over it. It actually works really well...Just don't stick your fingers together. Regular superglue can irritate your skin, so if you have time to plan ahead, buy some Dermabond, which is a cyanoacrylate surgical glue...They use it in place of stitches sometimes. Works great. Your wounds will NOT seep. Period.
Honestly though, people get cut all the time in professional kitchens, and I've never seen someone do more than slap some glue or a piece of tape over it, and keep going. You can't just quit work for a couple of days without throwing the whole business into chaos. It's a rough trade.