How to reduce the sweetness of a sauce?

Often my sauces are a tad too sweet. I usually use a base of onions, then some vegetables (spinach, leek, bell pepper, ...) + broth. This is sometimes blended, sometimes not.

I presume that the sweetness comes from the onions, so the obvious suggestion is to use less. I understand that and will try my best.

This being said, is there something I could add that would reduce the sense of sweetness? I tried salt and acids, but it helps so far only.

My question is probably: is there an antagonist of sugar in sauces?

If you want to reduce the sweetness of your sauce but otherwise leave the flavour profile basically the same, then your best bet is probably to reduce the percentage of ingredients that are high in sugar. Onions, leeks, and peppers are all high in sugar, and depending on how they are cooked, can be intensely sweet... so perhaps cooking technique is also something that could be altered?

If the horse has already bolted and the sauce is made but you want to reduce it's sweetness without altering it's flavour profile. Then dilution by adding water, or more of the non-sweet ingredients (spinach, broth, etc.) you used to make the sauce is probably your best bet. This isn't always possible though as, depending on the recipe and how finished the sauce is, you may essentially have to make a second sauce and mix the two together.

If you're open to altering the flavour profile, then I'd recommend researching Chinese and Thai sauces for inspiration. They're often high in added sugar and whilst they do still taste sweet, their sweetness is balanced with other flavours such as heat, acidity, saltiness, savoury umami depth from ingredients such as fish sauce, soy sauce etc.

Adding lime juice, chillies, and fish sauce obviously won't work well with all sauces but the basic principles of salty, savoury, acidic etc. are broadly applicable to cooking as a whole, even if you are using yeast extract instead of fish sauce for that salty, savoury depth.

If you could provide more details about your sauce, perhaps someone could give you a more tailored suggestion.