How do I salvage an onion gravy?

Salty and/or bitter flavours balance out sweetness. So if you add a bit of Marmite/Vegemite or similar, that would make the gravy less sweet, and also give it a 'deeper' flavour. (And if you don't like Marmite, don't worry, you won't taste it in the end result.) You can achieve similar result with a bit of stock/bouillon powder.

What is the base of your gravy? Water or milk? If it is milk, just pour some in and stir. It will cut the sweet taste.

Several things you might do.

One is cut down or eliminate the sweet red onions, and go for something mild but not so sweet. Vidalia onions are a good choice, but this is only in the US. See if your store can help you find a milder onion. .

I might use one red onion, and one white or yellow onion.

The broth concentrate found in the soup aisle is great, choose the beef one. It perks up the beef flavor. But beware, it is very salty. So if you have already salted your dish, don't use the broth concentrate.

YOu could add some cooked peppers, that would wake up the taste.

Increase the amount of seasonings.

I was always taught that red onion was used raw and cold for salads. My 7+ layer salad is the bomb with red onions, cut thin and small, and drowned in sugar/mayonaise dressing.