I’m surprised no one has mentioned the Vidalia Chop Wizard available at Walgreen’s, BB&B, WalMart, etc. DH and I have been cooking together for over 30 years and have all the usual cooking appliances and gadgets. I don’t go in much for the as-seen-on-TV stuff but was at a friend’s house a few years ago and watched her produce perfect tomato dice in seconds with this gadget. I had one less than an hour later and have never been without it. I use it ALL the time, onions are only part of its magic. It makes pico de gallo in minutes: 6 roma tomatoes, 1 jalapeno (or more) and half a small red onion diced with the Wizard, add some chopped cilantro, a dash of salt, and sprinkle with fresh lime juice. This is a staple in our house.
I use it to chop every vegetable imaginable, even carrots. Best ROI on any gadget we’ve ever purchased.