To caramelize the onions, melt the butter in a large skillet over low heat. Add the onions and toss with the sugar and salt. Allow to cook, stirring every 8-10 minutes, for about 40 minutes, or until onions are a uniform golden brown. Add the garlic and stir until fragrant. Place the onions in the slow cooker.
Turn the heat up to high and add the beef pieces, searing for a minute or two on each side. Deglaze the pan with the sherry, scraping up any browned bits with a spatula, then pour the mixture into the slow cooker.
Add the other vegetables, beef broth, bay leaf, and thyme to the slow cooker and cook on low for 8 hours or on high for 4 hours.
If you can't find prepackaged stew beef, feel free to substitute with chuck roast or pot roast sliced into chunks.