You might also change how you use the onion. When I'm making a chicken stock, I use one whole, unpeeled onion, cut it in half and blacken the cut faces in a very hot, dry pan. Really blacken the hell out of em and then throw em into the stock pot. Gives good colour and a good richness without being too sweet.

Embrace the inner sweetness of the vegetables!

A tablespoon of granulated white sugar contains 12.18 grams of carbohydrates, almost the same as a cup of chopped carrots (12.26 g); a cup of chopped onion contains 16.18 g. That's a whole lot of natural sugar going into your pot of soup! Learn to love it, or else figure out how much or what combination of wine or vinegar or sour citrus suits your palate: you certainly don't want to stop using the usual vegetables or you'll lose not only nutrients but also depth of flavor.

A squeeze of lemon or splash of champagne vinegar stirred in before serving would work, too, if the wine didn't do the trick. (I put vinegar on just about all of my soups in the winter, for just the reasons stated in this thread. Cooked carrots and cooked onions can make a soup or stew so, so sweet!!)

Wine! I will cut down on the carrots and onions and bump up the celery and wine. Many thanks to all....

I might be way off, but vegetables get sweeter as the climate gets colder; if you are using fresh local veggies and this sweetness has been getting stronger over the last month or so that may a factor. However if you are getting most veggies from the supermarket and therefore all over the world it would be hard to gauge. Are you taking any medication or has your diet changed that might be throwing off your palate?

I've also had soups turn out too sweet if I use too much onion. I second the white wine suggestion.

Carrots, onions, turnips and cabbage can all add a sweetness to soups and sometimes more than others. Have you tried adding a bit of dry white wine to counter the sweetness?